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 endpoint value coming from Jupyter config (e.g. [github_token](https://github.com/ReviewNB/jupyterlab-gitplus/blob/5a50bdede3637535c37bde49ad86b839ea00ee26/jupyterlab_gitplus/extension.py#L16)).
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 & this with endpoint value coming from Jupyter config (e.g. github_token).