Add support for git push options (git push -o <option>)

Gitlab supports git push options, for example I can run git push -o ci.skip to push a single commit without triggering the CI. It would be handy if Travis-CI also supported this.

See https://docs.gitlab.com/ee/user/project/push_options.html

I don’t think that’s something we can support without a lot of help from GitHub; GitHub will have to recognize this kind of push event, and emit the event we can use. Then, we can respond to such an event.

What you can do right now instead is to sent a POST request to do the same thing.