Add support for alternate screen buffers

Closes #250

src/Spectre.Console/Capabilities.cs

@@ -55,6 +55,12 @@ namespace Spectre.Console
         /// </summary>
         public bool Unicode { get; set; }
 
+        /// <summary>
+        /// Gets or sets a value indicating whether
+        /// or not the console supports alternate buffers.
+        /// </summary>
+        public bool AlternateBuffer { get; set; }
+
         /// <summary>
         /// Initializes a new instance of the
         /// <see cref="Capabilities"/> class.