configurable response on registration - proposal for #26

- new message for clients they registered
- new variables that allow disable public response on user registration
get boolean from announce_registration config and extend error handling

fixes commit 59e0b66 for #27
[ADMIN] changes to the !register command behavior and settings :
- add announce_registration option to config to (de)activate public announcement of player using the !register command
- !register command feedback message is now configurable in config. See regme_confirmation