Use the version of esprima that parses `export default` correctly.

This is the same version used by the es6-module-transpiler.
Add bin/esnext to make experimenting with it on the command-line easier.

This also will hopefully enable build pipelines that are based on, say, make or rake.
Don't badly duplicate the list of options in both compile and transform.
Add a section to the README on using the CLI.
Update the Acknowledgements section to add Ariya & esprima.