Add lineman build task to copy generated files

* Override `copy:dev` to copy generated assets to `public/`
* Add a task to `watch` to run `copy:dev` when any of the generated assets change.

This is to address issues in development with: https://github.com/exercism/exercism.io/commit/65e9b45a5469df8e13a0146fd82f42bcb5fb1066

Seems to work locally when I change some css. With this fix I _think_ we can delete the committed assets in `public`.