Support checking multiple directories

I'm currently moving towards using a theme system for my templates and static files (https://github.com/voxpelli/node-tema) and that means that a static file can be found either in the active theme or in a parent of that theme – which means that there might be more than one directory that needs to be searched for a file to be found.

This pull request enables this module to look for a file in multiple directories and is fully backwards compatible with defining just a single directory as well.