Generalized mkExe.hs to work with other Elm files

Renamed Cabal name to ElmIO because elm-io didn't work with the
generated Paths_ module

Added prescript.js and handler.js to data-files, for programmatic access
to them

Changed CLI to accept an infile (Elm source file) and outfile (generated
js file to run with node)
Make IO directory runtime accessible
Add some error checking for arg handler