Support Gitlab (and other github-like backends)

Hi,

How hard would it be to have a pluggable backend to switch from github to gitlab if desired?
I guess most of the logic behind github-changelog-generator is not github specific but solely relies on the fact that there is issues, PR, tags on them and so on.
Gogs could be another candidate for that.

I would like to help, but I'm not very familiar with Ruby so I really couldn't handle the refactoring needed to have this pluggable architecture I think…

See also https://gitlab.com/gitlab-org/gitlab-ce/issues/15345 for the need, already expressed at gitlab (without so much news for some time).