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/Console/Calendars/Program.cs

@@ -0,0 +1,19 @@
+using Spectre.Console;
+
+namespace Calendars
+{
+    public static class Program
+    {
+        public static void Main(string[] args)
+        {
+            AnsiConsole.WriteLine();
+            AnsiConsole.Render(new Calendar(2020, 10)
+                    .RoundedBorder()
+                    .HighlightStyle(Style.Parse("red"))
+                    .HeaderStyle(Style.Parse("yellow"))
+                    .AddCalendarEvent("An event", 2020, 9, 22)
+                    .AddCalendarEvent("Another event", 2020, 10, 2)
+                    .AddCalendarEvent("A third event", 2020, 10, 13));
+        }
+    }
+}