HipChat: Add boolean to ignore forks and watches

Take 2.

I'd like to offer a way to quite those dang "fork" and "watch" events. For the projects I work on, they become noise in a HipChat room where we're trying to get meaningful stuff done and discussed. It's important to me that we have less of this noise.

In #487, I proposed removing them altogether, but @technoweenie thought they had better check with the HipChat staff, and it was ultimately denied. This is a compromise: allow users (on the GitHub side) to quiet these events by selecting a checkbox. This pull request proposes an opt-out solution, keeping the default the same, but allowing for a little flexibility and noise-reduction capabilities.

Let me know what you think -- I'm definitely eager to have this as an option, at least.