Add --only-dependencies

[wolverian]

Reason: I want to avoid the slow rebuilding of dependencies on each build on a CI service.

Motivation: I use CircleCI.com as a CI service. Each build is started with a clean disk by default.

They support caching dependencies. I can use elm package install to install dependencies and cache them, but that doesn't actually build the dependencies, so it caches just the sources.

I want to cache the built dependencies too.

[wolverian]

Additional note: you can of course just build the project and then cache the dependencies, but that might lead to caching too much (i.e. I don't want to dirty the cache with builds of my own sources).

[jvoigtlaender]

@wolverian, doesn't elm-make without parameters do exactly what you want?

[evancz]

Tracking from meta issue #98