Allow away response for "@username" format (not just "username:")

Currently, hubot won't reply with away status if this is used:

    @sally what are you working on?

Might also be nice to response with status when "@" is used, even if the person is not being addressed directly. Perhaps this could be toggleable, as it might be too much noise.

    I think @sally is working on it...?

cc: @anandpathaksharma