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/Rendering/Borders/TableBorderPart.cs

@@ -0,0 +1,98 @@
+namespace Spectre.Console.Rendering
+{
+    /// <summary>
+    /// Represents the different parts of a table border.
+    /// </summary>
+    public enum TableBorderPart
+    {
+        /// <summary>
+        /// The top left part of a header.
+        /// </summary>
+        HeaderTopLeft,
+
+        /// <summary>
+        /// The top part of a header.
+        /// </summary>
+        HeaderTop,
+
+        /// <summary>
+        /// The top separator part of a header.
+        /// </summary>
+        HeaderTopSeparator,
+
+        /// <summary>
+        /// The top right part of a header.
+        /// </summary>
+        HeaderTopRight,
+
+        /// <summary>
+        /// The left part of a header.
+        /// </summary>
+        HeaderLeft,
+
+        /// <summary>
+        /// A header separator.
+        /// </summary>
+        HeaderSeparator,
+
+        /// <summary>
+        /// The right part of a header.
+        /// </summary>
+        HeaderRight,
+
+        /// <summary>
+        /// The bottom left part of a header.
+        /// </summary>
+        HeaderBottomLeft,
+
+        /// <summary>
+        /// The bottom part of a header.
+        /// </summary>
+        HeaderBottom,
+
+        /// <summary>
+        /// The bottom separator part of a header.
+        /// </summary>
+        HeaderBottomSeparator,
+
+        /// <summary>
+        /// The bottom right part of a header.
+        /// </summary>
+        HeaderBottomRight,
+
+        /// <summary>
+        /// The left part of a cell.
+        /// </summary>
+        CellLeft,
+
+        /// <summary>
+        /// A cell separator.
+        /// </summary>
+        CellSeparator,
+
+        /// <summary>
+        /// The right part of a cell.
+        /// </summary>
+        CellRight,
+
+        /// <summary>
+        /// The bottom left part of a footer.
+        /// </summary>
+        FooterBottomLeft,
+
+        /// <summary>
+        /// The bottom part of a footer.
+        /// </summary>
+        FooterBottom,
+
+        /// <summary>
+        /// The bottom separator part of a footer.
+        /// </summary>
+        FooterBottomSeparator,
+
+        /// <summary>
+        /// The bottom right part of a footer.
+        /// </summary>
+        FooterBottomRight,
+    }
+}