GitHub plugin install + per-plugin updates

[MichaelCosby]

This one lets you install plugins via github. It's really useful for me since my install of clideck is inside a remote-hosted docker container, but I imagine it would make plugin install easier for pretty much everyone. Also, knowing where the plugin came from meant I could add in an ability to download plugin updates. I did write it so that it could allow multiple plugins in a single repo,I don't know if you'd object to that or not. This is another one that's already implemented in my branch. Here's a screenshot: