Allow custom help providers (#1259)

Allow custom help providers

* Version option will show in help even with a default command

* Reserve `-v` and `--version` as special Spectre.Console command line arguments (nb. breaking change for Spectre.Console users who have a default command with a settings class that uses either of these switches).

* Help writer correctly determines if trailing commands exist and whether to display them as optional or mandatory in the usage statement.

* Ability to control the number of indirect commands to display in the help text when the command itself doesn't have any examples of its own. Defaults to 5 (for backward compatibility) but can be set to any integer or zero to disable completely.

* Significant increase in unit test coverage for the help writer.

* Minor grammatical improvements to website documentation.

src/Spectre.Console.Testing/FakeTypeResolver.cs

@@ -35,10 +35,12 @@ public sealed class FakeTypeResolver : ITypeResolver
         }
 
         if (_registrations.TryGetValue(type, out var registrations))
-        {
+        {
+            // The type might be an interface, but the registration should be a class.
+            // So call CreateInstance on the first registration rather than the type.
             return registrations.Count == 0
-                ? null
-                : Activator.CreateInstance(type);
+                ? null
+                : Activator.CreateInstance(registrations[0]);
         }
 
         return null;