Extra paths

This pull request adds the ability to add arbitrary extra paths to the paths provided to FPM as input when using Omnibus packages.

The reason for this is so you can do things like include init scripts and config files into a package - when you're using omnibus_dir and not a destdir, you lose this ability.

I realise I could just merge this as it's fairly trivial, but I just wanna check it's okay with you (it's your project, after all!)