Adding git repos to template directory

[frankturnerv]

Feature Request

Our team uses github for compiled. It would be nice to be able to link to a repo on GitHub instead of my local machine.

[AndersonReyes]

@timkpaine How you think this should be handled? via a different config list of repos like c.JupyterTemplates.git_repos = [] or or use the same template_dirs list but deetect git repos with the ".git" suffix? like

template_dirs = ["/usr/loca/share/templates", "https://github.com/jpmorganchase/jupyterlab_templates.git"]

and have a separate git template loader handle those to git clone