Make linuxdeployqt -g aware of feature branches and PRs

From https://github.com/AppImage/AppImageKit/issues/518#issuecomment-342332710 @dirkhh

> one thing that I find challenging... if I want to play with this on a feature branch (I'm working on debugging a problem, I need AppImages to deal with that)... if I push to that branch, that becomes the latest pre-release binary.
Would it be possible to use continuous for master and continuous-branchname for other branches and offer them separately? Or is this beyond what GitHub/Travis can do?