Add Spectre.Cli to Spectre.Console

* Renames Spectre.Cli to Spectre.Console.Cli.
* Now uses Verify with Spectre.Console.Cli tests.
* Removes some duplicate definitions.

Closes #168

examples/Cli/Dynamic/MyCommand.cs

@@ -0,0 +1,20 @@
+using System;
+using Spectre.Console.Cli;
+
+namespace Dynamic
+{
+    public sealed class MyCommand : Command
+    {
+        public override int Execute(CommandContext context)
+        {
+            if (!(context.Data is int data))
+            {
+                throw new InvalidOperationException("Command has no associated data.");
+                
+            }
+
+            Console.WriteLine("Value = {0}", data);
+            return 0;
+        }
+    }
+}