Optionally only build packages in production

Precompiling for production should not compile all separate files.
When setting `:production_packages_only` to true in the Sinatra app,
the `Builder` will not build files if `RACK_ENV` is `production`.

I was not quite sure how to access the sinatra settings from the builder.
The suggested implementation works, but I would love your suggestion on this. :-)

Cheers!