add whitelist_javascript_paths to papers config, exclude these directories from the check

We're using gulp as a build tool in our project and would love to exclude the files on a certain path.  This patch adds an attribute to the config that contains a list of paths to exclude from validation for javascript directories.

Thanks!
We could probably just do a `files.reject! { |file| file.start_with?(whitelist_dir) }` instead of making these checks?