Make it easy to thread limbo responses

[llimllib]

Right now, to thread responses from limbo plugins you have to revert from returning text to using slack.post_message. It would probably be better if plugins could return a (message, timestamp) tuple to thread responses?

[llimllib]

example problem: the !weather plugin will not respond in a thread

[llimllib]

Greatly improved in 020a961 , but could still have a better interface