Don't limit tables and grids to markup text

Closes #13

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

@@ -0,0 +1,14 @@
+namespace Spectre.Console.Rendering
+{
+    /// <summary>
+    /// Represents an invisible border.
+    /// </summary>
+    public sealed class NoBorder : Border
+    {
+        /// <inheritdoc/>
+        protected override string GetBoxPart(BorderPart part)
+        {
+            return " ";
+        }
+    }
+}