Add support for markdown tables

Closes #85

* Split Border into BoxBorder and TableBorder
* Change how different table parts are composed
* Add markdown table border

src/Spectre.Console/TableBorder.cs

@@ -0,0 +1,157 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Text;
+using Spectre.Console.Internal;
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Represents a border.
+    /// </summary>
+    public abstract partial class TableBorder
+    {
+        private readonly Dictionary<TableBorderPart, string> _lookup;
+
+        /// <summary>
+        /// Gets a value indicating whether or not the border is visible.
+        /// </summary>
+        public virtual bool Visible { get; } = true;
+
+        /// <summary>
+        /// Gets the safe border for this border or <c>null</c> if none exist.
+        /// </summary>
+        public virtual TableBorder? SafeBorder { get; }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="TableBorder"/> class.
+        /// </summary>
+        protected TableBorder()
+        {
+            _lookup = Initialize();
+        }
+
+        private Dictionary<TableBorderPart, string> Initialize()
+        {
+            var lookup = new Dictionary<TableBorderPart, string>();
+            foreach (TableBorderPart? part in Enum.GetValues(typeof(TableBorderPart)))
+            {
+                if (part == null)
+                {
+                    continue;
+                }
+
+                var text = GetBorderPart(part.Value);
+                if (text.Length > 1)
+                {
+                    throw new InvalidOperationException("A box part cannot contain more than one character.");
+                }
+
+                lookup.Add(part.Value, GetBorderPart(part.Value));
+            }
+
+            return lookup;
+        }
+
+        /// <summary>
+        /// Gets the string representation of a specific border part.
+        /// </summary>
+        /// <param name="part">The part to get a string representation for.</param>
+        /// <param name="count">The number of repetitions.</param>
+        /// <returns>A string representation of the specified border part.</returns>
+        public string GetPart(TableBorderPart part, int count)
+        {
+            // TODO: This need some optimization...
+            return string.Join(string.Empty, Enumerable.Repeat(GetBorderPart(part)[0], count));
+        }
+
+        /// <summary>
+        /// Gets a whole column row for the specific column row part.
+        /// </summary>
+        /// <param name="part">The column row part.</param>
+        /// <param name="widths">The column widths.</param>
+        /// <param name="columns">The columns.</param>
+        /// <returns>A string representing the column row.</returns>
+        public virtual string GetColumnRow(TablePart part, IReadOnlyList<int> widths, IReadOnlyList<IColumn> columns)
+        {
+            if (widths is null)
+            {
+                throw new ArgumentNullException(nameof(widths));
+            }
+
+            if (columns is null)
+            {
+                throw new ArgumentNullException(nameof(columns));
+            }
+
+            var (left, center, separator, right) = GetTableParts(part);
+
+            var builder = new StringBuilder();
+            builder.Append(left);
+
+            foreach (var (columnIndex, _, lastColumn, columnWidth) in widths.Enumerate())
+            {
+                var padding = columns[columnIndex].Padding;
+                var centerWidth = padding.Left + columnWidth + padding.Right;
+                builder.Append(center.Multiply(centerWidth));
+
+                if (!lastColumn)
+                {
+                    builder.Append(separator);
+                }
+            }
+
+            builder.Append(right);
+            return builder.ToString();
+        }
+
+        /// <summary>
+        /// Gets the string representation of a specific border part.
+        /// </summary>
+        /// <param name="part">The part to get a string representation for.</param>
+        /// <returns>A string representation of the specified border part.</returns>
+        public string GetPart(TableBorderPart part)
+        {
+            return _lookup[part].ToString(CultureInfo.InvariantCulture);
+        }
+
+        /// <summary>
+        /// Gets the character representing the specified border part.
+        /// </summary>
+        /// <param name="part">The part to get the character representation for.</param>
+        /// <returns>A character representation of the specified border part.</returns>
+        protected abstract string GetBorderPart(TableBorderPart part);
+
+        /// <summary>
+        /// Gets the table parts used to render a specific table row.
+        /// </summary>
+        /// <param name="part">The table part.</param>
+        /// <returns>The table parts used to render the specific table row.</returns>
+        protected (string Left, string Center, string Separator, string Right)
+            GetTableParts(TablePart part)
+        {
+            return part switch
+            {
+                // Top part
+                TablePart.Top =>
+                    (GetPart(TableBorderPart.HeaderTopLeft), GetPart(TableBorderPart.HeaderTop),
+                    GetPart(TableBorderPart.HeaderTopSeparator), GetPart(TableBorderPart.HeaderTopRight)),
+
+                // Separator between header and cells
+                TablePart.Separator =>
+                    (GetPart(TableBorderPart.HeaderBottomLeft), GetPart(TableBorderPart.HeaderBottom),
+                    GetPart(TableBorderPart.HeaderBottomSeparator), GetPart(TableBorderPart.HeaderBottomRight)),
+
+                // Bottom part
+                TablePart.Bottom =>
+                    (GetPart(TableBorderPart.FooterBottomLeft), GetPart(TableBorderPart.FooterBottom),
+                    GetPart(TableBorderPart.FooterBottomSeparator), GetPart(TableBorderPart.FooterBottomRight)),
+
+                // Unknown
+                _ => throw new NotSupportedException("Unknown column row part"),
+            };
+        }
+    }
+}