Allow installing from github

I'd like to do:

```
stack install https://github.com/ndmitchell/hoogle.git
```

And have that "just work". Bonus points if the version without `.git` also works. In particular, this would need to check out the repo, if a `stack.yaml` was missing it would need to `stack init`, and then `stack install`. See https://hackage.haskell.org/package/cabalg by @dmaliko for a version for Cabal.