[DO NOT MERGE] Add admin plugin

[dashezup]

should already works, but some codes may could be improved

-- need to deal with anonymous admin

I feel it's a bit messy (to check if the user is in the group, if it's admin etc.)

[dashezup]

I can't think of a good way to deal with this, so I'll leave it for now.

maybe it's a bad idea to accept both !command @username and !command (as a reply).