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/Extensions/HasBoxBorderExtensions.cs

@@ -0,0 +1,101 @@
+using System;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Contains extension methods for <see cref="IHasBoxBorder"/>.
+    /// </summary>
+    public static class HasBoxBorderExtensions
+    {
+        /// <summary>
+        /// Do not display a border.
+        /// </summary>
+        /// <typeparam name="T">An object type with a border.</typeparam>
+        /// <param name="obj">The object to set the border for.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static T NoBorder<T>(this T obj)
+            where T : class, IHasBoxBorder
+        {
+            return SetBorder(obj, BoxBorder.None);
+        }
+
+        /// <summary>
+        /// Display a square border.
+        /// </summary>
+        /// <typeparam name="T">An object type with a border.</typeparam>
+        /// <param name="obj">The object to set the border for.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static T SquareBorder<T>(this T obj)
+            where T : class, IHasBoxBorder
+        {
+            return SetBorder(obj, BoxBorder.Square);
+        }
+
+        /// <summary>
+        /// Display an ASCII border.
+        /// </summary>
+        /// <typeparam name="T">An object type with a border.</typeparam>
+        /// <param name="obj">The object to set the border for.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static T AsciiBorder<T>(this T obj)
+            where T : class, IHasBoxBorder
+        {
+            return SetBorder(obj, BoxBorder.Ascii);
+        }
+
+        /// <summary>
+        /// Display a rounded border.
+        /// </summary>
+        /// <typeparam name="T">An object type with a border.</typeparam>
+        /// <param name="obj">The object to set the border for.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static T RoundedBorder<T>(this T obj)
+            where T : class, IHasBoxBorder
+        {
+            return SetBorder(obj, BoxBorder.Rounded);
+        }
+
+        /// <summary>
+        /// Display a heavy border.
+        /// </summary>
+        /// <typeparam name="T">An object type with a border.</typeparam>
+        /// <param name="obj">The object to set the border for.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static T HeavyBorder<T>(this T obj)
+            where T : class, IHasBoxBorder
+        {
+            return SetBorder(obj, BoxBorder.Heavy);
+        }
+
+        /// <summary>
+        /// Display a double border.
+        /// </summary>
+        /// <typeparam name="T">An object type with a border.</typeparam>
+        /// <param name="obj">The object to set the border for.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static T DoubleBorder<T>(this T obj)
+            where T : class, IHasBoxBorder
+        {
+            return SetBorder(obj, BoxBorder.Double);
+        }
+
+        /// <summary>
+        /// Sets the border.
+        /// </summary>
+        /// <typeparam name="T">An object type with a border.</typeparam>
+        /// <param name="obj">The object to set the border for.</param>
+        /// <param name="border">The border to use.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static T SetBorder<T>(this T obj, BoxBorder border)
+            where T : class, IHasBoxBorder
+        {
+            if (obj is null)
+            {
+                throw new ArgumentNullException(nameof(obj));
+            }
+
+            obj.Border = border;
+            return obj;
+        }
+    }
+}