Better support for locally installed tools

[robyoder]

The --elm-test arg was only working with absolute paths, and a locally installed copy of elm wouldn't work at all.

Using path.resolve seemed like a pretty straightforward fix for elm-test, but then I realized the default value for the arg was "elm-test", which is definitely not meant to be resolved as a relative path. I'm not sure what the cleanest way to approach this would be, but I took the perspective that anyone setting it to something other than the default is most likely to be using a path, not another name meant to be resolved by Node, so we can just resolve the value as a relative path if it's not the default value.

For elm, I noticed in #20 that @zwilias had recommended using npm-which, and that did the trick nicely.

I've not added tests because it's getting late and I'm just trying to get this working for my project and I wasn't actually totally sure how to test the elm one anyway. I'd love any guidance on this.

Resolves #45

[robyoder]

I wonder if we should use npm-which on the default elm-test value also, to provide default support for local installs.

[robyoder]

Hey, @zwilias, can you take a look?