WIP Allow installation of local folders as packages

[kasbah]

Just started hacking on this a bit as it was annoying me again. The idea is for elm-package install foo
to check if foo is a directory and then install it if it is. The dependencies in elm-package.json would look something like:

    "dependencies": {
        "elm-lang/core": "1.1.1 <= v < 2.0.0",
        "local:///home/kaspar/elm-project/foo": "1.0.0 <= v < 2.0.0"
    }

Looking for any comments in general at this point. core is kind of hard coded even more than the rest so it'll have to be pried out a bit (maybe a switch --local-core ?) before this begins to solve #91.

[evancz]

I don't think it makes sense to keep the WIP open on this repo.