Debugger combined call

I've created `Elm.debugFullscreen()` which does not take ports, since they're unsupported with debugging. It stores the filename it receives in the `Elm.Debugger` object for the debugging interface to retrieve.
Creating an object like `Debug` for `Debug.fullscreen()` seems like it would be an intrusive change.

Motivated by https://github.com/elm-lang/elm-server/pull/12.