Add multi-upstream set-upstream support

[alanpearce]

I think it would be useful for some projects to have the possibility of being able to run gh push -u origin,github, such that subsequent runs of gh push do the equivalent of gh push origin,github, in the same manner that git push -u works with single upstreams.

I can try implementing this, but I'm not quite sure what approach to take with storing the upstream data.

[owenthereal]

This is a good idea. Note that moving forward gh and hub will be merged. Any future contribution will go to github/hub