Support GitLab

[jayvdb]

With the big migration from GitHub to GitLab, it would be helpful if yehua supported GitLab.

First step is adding a question for scm_host, and slightly changing the organisation question.

Ideally generate GitLabs CI's .gitlab-ci.yml file.

Also it is possible to support Shippable, which works with GitLab, by ensuring the .travis.yml file works correctly on Shippable.

[chfw]

I may add a question allow developer switch between gitlab+gitlabci and github+travis-ci.

[chfw]

I am not using gitlab so I would appreciate some gitlab user to contribute.