Allow handlers to pass raw events to callbacks

[ryan-lane]

Currently omnibot always strips and parses incoming events. We should store the raw event on the object, and have a callback config option that allows the raw message to be sent to the backend. Option should likely be: parse, strip_sensitive, raw or something along those lines.

Doing so would allow omnibot to be used as a generic proxy, rather than requiring backend be omnibot-specific.