Fix https://github.com/facebook/regenerator/issues/137.

Use commoner instead of commander so it is trivial to transform
an entire directory of .js files from the command line.

The traditional `regenerator --include-runtime es6.js > es5.js` use
case still works, as well.
This looks great! The only thing I'm going to change is this `^` (to a `~`) because `npm` doesn't understand `^` if you're using an older version of Node.