Add bin for creating index files

Hi there,

I thought it would be handy to have a bin file which allows you to generate a `index.js` into the CWD. The `index.js` being the file that requires the directory it resides in and exports it.

I didn't bother with tests yet, since I first wanted to know what you think. If you like the idea I can add the test, update the readme and squash the commits.