Add support for commit status contexts

[farmdawgnation]

Hi there,

With hub4j/github-api#97 the GitHub Java API now supports commit status contexts. Can we get support for a configuration option that would allow ghprb to push statuses from particular jobs with a particular context?

So on a per-job basis you could specify what the context should be on the commit status. This is useful for cases like at my employer where ghprb may trigger multiple jenkins jobs for different purposes (e.g. one for units, one for integrations... or some similar configuration).

If I get time today I'm going to try and build this out myself. But if someone else would like to take a stab at it then go for it.

[farmdawgnation]

(This is currently blocked on a new release of the github-api jenkins plugin, too, which I've already pinged the maintainer about. Going to still try and get it working locally with a snapshot build then move from there.)