Let agent99 respond to channel invitations

Implements an "RSVP" plugin so that agent99 can respond to requests to
join or leave channels.

To invite agent99 to a channel, ask her like this:
```
agent99 pls2join #my-cool-channel
```

If you get sick of agent99, ask her to leave:
```
agent99 pls2leave
```
More succinct syntax for channel eviction
Merge pull request #45 from 99designs/features/plugins/rsvp

Let agent99 respond to channel invitations