Add soundboard commands

[VoightKampff]

I would be nice to have sound board commands, i.e. being able to define "shortcuts" for video urls. The set of commands I would consider minimal is as follows:

1. !soundboard name url - define and re-define/overwrite a soundboard command
2. !soundboard name - invoke a soundboard command
3. !soundboard --delete name - delete a soundboard command
4. !soundboard --list - list soundboard commands

The name of the command could also be shortened to just !sb.