Generate working exercises with `makews.js`

Hi, @rvagg!

This pull request does a few things with the `makews.js` script:

1. Moves it from `util/makews.js` to `bin/makews.js` because having a `util/` directory and a `utils.js` module both in the project root is confusing and makes for more difficult tab completion.
2. Adds the `--force` argument to overwrite exercises generated. Nothing fancy here. You need to send the script *exactly* `--force` in *exactly* `process.argv[3]` or it will do *exactly* nothing.
3. Changes it to generate working exercises that require and create a `workshopper-exercise`. Added `workshopper-exercise` to `devDependencies` for that reason.
4. I also ran it with the bundled example exercises to update them.

I've tried to imitate your style of code as much as I could (which is always an interesting little adventure).

Again, thanks a bunch for this thing!