tmp untested Convert IRCBot to proper unloadable plugin

Finally it's possible to
1) start program without loading IRCBot plugin
2) stop without getting SIGINT in random places inside plugins
3) properly unload plugins on stop
4) configure IRCBot reconnect delay

Downsides:
1) Not sure how being not in main Ruby thread will affect
performance and latency.
2) My braindead PluginManager implementation completely fails at
initializing plugins dependency order. Since now IRCBot is no longer
guaranteed to be started last, it may lead to some spectacular fails.
Improve config help for IRCBot plugin
Add bundle install mention to README