allow per-channel key to be specified for IRC notifications

This should fix https://github.com/travis-ci/travis-ci/issues/1628 by allowing the specification of an IRC key on a per-channel basis, by appending ",$key" to the channel definition:

```
 irc:
    channels:
      - irc.oftc.net#akerl,sekrit
      - irc.freenode.net#coolkids,password
```

I chose "," as the delimiter because, as per my understanding, it will not occur in an IRC channel name.