Some Minor Enhancements and Bug Fixes

The builder now writes to the `build.{css,js}` files once it finishes building (since the assets were getting copied, it made sense to have the js/css made more "permanent" as well)

Speaking of assets, the setup before didn't work quite as I expected. In my case, for a mounted application, the assets wound up getting copied to the root dir, rather than the "out" dir. This is because `opts.out` was coming out blank in my case. Now, `opts.out` can be customized, as well as having the static middleware use the correct dir.

Lastly, just a minor change, dropped the use of `locals` as it's not really necessary.

Let me know what you think, I've kept each change in a separate commit so they can easily be excluded.