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/IHasTableBorder.cs

@@ -0,0 +1,13 @@
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Represents something that has a border.
+    /// </summary>
+    public interface IHasTableBorder : IHasBorder
+    {
+        /// <summary>
+        /// Gets or sets the border.
+        /// </summary>
+        public TableBorder Border { get; set; }
+    }
+}