Create Elm.gitignore

.gitignore for Elm projects. 

Ignores installed packages (elm-package.json) and elm-stuff/, as these files will be generated once elm-package has been run for a .elm file. This is also beneficial where dependencies have changed, and an install package is no longer required.

elm-repl generated files have also been ignored. Most of the time, elm-repl manages to delete these files automatically, but there are scenarios where these files are not deleted on the closing of elm-repl