Install plugin using GitHub repo URL

[ryanrolds]

I've been in situations where I'm working on a project and want to install/update plugins from a GH repo. Often these plugins are not ready for submission to the central plugin repo.

Example:
$ adapt install https://github.com/irisEd/adapt-irised-pathways.git

$ adapt install https://github.com/irisEd/adapt-irised-pathways.git#somebranch

It would great to store the URL in adapt.json so that adapt install would install from the GH repo as well.

{
    "dependencies": {
        "adapt-contrib-text": "*",
...
        "adapt-irised-pathways": "https://github.com/irisEd/adapt-irised-pathways.git#somebranch"
    }
}