Add GVA credit to tweets

[elliotblackburn]

Gun Violence Archive would appreciate a shout out in the tweets. Preferably something like the example below, we need to be aware of the 140 character limit though.

This would be good for a beginner, it could simply be added in the TwitterReporter for now. Alternatively we could add it to the Incident class and pass it though.

Example they suggested:

Source: GVA