`elm.codegen.json` local codegen-helpers requires `src\\` on Windows

[tankorsmash]

Thank you for the library!

Likely related to #59, the docs show src/ as a way to include all your src folder. If I do this on Windows, elm-codegen runs without error, but doesn't actually generate any files.

---

However, if I've got a file in src called Foo.elm, 'local': [ 'src/Foo.elm' ] successfully generates Gen/Foo.elm.

In order for me to get all my src files generated, I need to use two backslashes:

{
    "elm-codegen-version": "0.5.0",
    "codegen-helpers": {
        "packages": {
            "elm/core": "1.0.5"
        },
        "local": [
            "src\\"
        ]
    }
}

Which works after a short processing delay. Note the backslash too.

Thanks again for the library!