Add the ability to restrict channel output per-plugin

[rikai]

This may need to be a plugin-level thing rather than a global-level thing, or perhaps both.

For example, the twitter plugin currently outputs to all channels for all users, i'd like to be able to tell it to only output to certain channels for specific twitter users, but it could be useful to also have a high-level "only output to this channel", which can be overridden per-user.