Don't limit tables and grids to markup text

Closes #13

src/Spectre.Console/Rendering/Borders/RoundedBorder.cs

@@ -0,0 +1,37 @@
+using System;
+
+namespace Spectre.Console.Rendering
+{
+    /// <summary>
+    /// Represents a rounded border.
+    /// </summary>
+    public sealed class RoundedBorder : Border
+    {
+        /// <inheritdoc/>
+        protected override string GetBoxPart(BorderPart part)
+        {
+            return part switch
+            {
+                BorderPart.HeaderTopLeft => "╭",
+                BorderPart.HeaderTop => "─",
+                BorderPart.HeaderTopSeparator => "┬",
+                BorderPart.HeaderTopRight => "╮",
+                BorderPart.HeaderLeft => "│",
+                BorderPart.HeaderSeparator => "│",
+                BorderPart.HeaderRight => "│",
+                BorderPart.HeaderBottomLeft => "├",
+                BorderPart.HeaderBottom => "─",
+                BorderPart.HeaderBottomSeparator => "┼",
+                BorderPart.HeaderBottomRight => "┤",
+                BorderPart.CellLeft => "│",
+                BorderPart.CellSeparator => "│",
+                BorderPart.CellRight => "│",
+                BorderPart.FooterBottomLeft => "╰",
+                BorderPart.FooterBottom => "─",
+                BorderPart.FooterBottomSeparator => "┴",
+                BorderPart.FooterBottomRight => "╯",
+                _ => throw new InvalidOperationException("Unknown box part."),
+            };
+        }
+    }
+}