[CHEF-5021] Include "extra_package_files" var which adds to install_path

This includes the ability to include files/folders outside of
/opt/{project} that you may want included in the full package.

https://tickets.opscode.com/browse/CHEF-5021

Example of the usage in a project.rb file.
https://github.com/petecheslock/sensu-build/blob/convert-to-omnibus/config/projects/sensu.rb#L26-L37