0.0.2: Refactor into a plugin system

The previous hoary "source" paradigm was too unwieldy. The bot now uses
"plugins" which define commands.  The command prefix is defined
separately now so it can be changed if necessary.  Overall, things are
a bit less labyrinthine.

It's not quite completely general yet, but good enough for now.

Old sources.js has been split into plugin.watchlist and plugin.crawl.

The only actual new feature is more precise filtering of ##crawl relay
messages. (Closes #3.)

Bumped version to 0.0.2.