Improve the way the command registry handles commands

[Crsi]

The registry should fulfill the following requirements in any way:

1. Singleton-like behavior (e.g. prohibit two different instances of a command)
2. Allow parsing and the generation of help & usage messages (e.g. don't do silly stuff)
3. Easy to use (e.g. adding and removing commands and so on is simple enaugh)
4. Divide the command executors from the Telegram updater or handler

[Crsi]

The new mate_bot.registry module addressed this issue. It looks like it solves the issue gracefully.