Support more than just a single Github repository

[ianshward]

Dispatch should allow for dispatch messages to define to what Github repository a message gets posted, instead of having this be a single repository. This entails at least changing the message spec. to support a repo parameter in the github property, and having the code use this to determine where to post the message.

[k-mahoney]

Discussion moved to https://github.com/mapbox/dispatch/issues/98