Trying to support alternative git servers

[tschm]

Generalize the _split_repo_url. Still works as exactly as before for GitHub urls but doesn't fall over for GitLab.

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[tschm]

I was hoping I could point to our internal Jupyter-Hub server. There is no chance I can rely on Binder.

[choldgraf]

Gotcha - can you confirm that this actually works for GitLab or other repositories? Could you also add some test or example in the documentation that shows a functioning GitLab link (at least with mybinder.org, I know it's hard to test against a JupyterHub)