Consolidate clean commands

[emmiegit]

There are a number of specialized cleanup requests, such as for a user across the guild, or within certain time slices. There are already a bit too many cleanup variants, it would be better if we had one cleanup command that accepted arguments instead.

For instance !cleanup [--count NUM] [--channel CHAN | --global] [--user USER] [--from ID/DATE] [--to ID/DATE]