amit2rrr

Currently GitPlus only works with github.com. We should extend it to work with GitHub Enterprise. The main step here would be to replace hardcoded URL like [this](https://github.com/ReviewNB/jupyterlab-gitplus/blob/master/jupyterlab_gitplus/github_v3.py#L10) & [this](https://github.com/ReviewNB/jupyterlab-gitplus/blob/master/jupyterlab_gitplus/handlers.py#L20) with...