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 gtfo
```

@richo I assume this will work. I may need your help to test locally.