Change Working Directory option

[jcollard]

With the new elm-make integration the src-dir flags don't really make sense.

To integrate elm-repl with elm-mode in emacs in an intelligent way, I think it makes sense to add a :cd option to allow switching between projects easily without needing to launch multiple processes of elm-repl or restarting elm-repl completely.

[evancz]

I think the :command pattern is pretty inelegant. How is this done in the Python or ruby repl?

[jcollard]

In python there is os.chdir(path) which is used by the python-mode installed on my system.