[Draft] Add new message.CommandName filters

[PaulSonOfLars]

What

This change adds a few new message filters to allow MessageHandlers to function similarly to CommandHandlers. With this new change, a CommandHandler can be considered a specialised MessageHandler (with command filters pre-applied).

Worth noting that with this change, the CommandHandler type becomes significantly less useful; maybe the type should be removed, and the NewCommand method should be left simply as a wrapper around NewMessage?

Impact

• Are your changes backwards compatible? Yes; unless we choose to remove the CommandHandler type.
• Have you included documentation, or updated existing documentation? Yes
• Do errors and log messages provide enough context? Yes