Add option to display location of elm_runtime.js

Fixes my part of https://github.com/elm-lang/Elm/issues/492 .

I needed this in order to port the IO stuff to be pure node (no Haskell dependency unless you build elm/elm-get) using https://github.com/arsatiki/elmloader .

I'm not particularly attached to the `--show-runtime` name but I am in need of the functionality.