Allow mutliple spec_dir entries

The primary motivation here is to add support for alternative asset path setups as in [sections_rails](https://github.com/kevgo/sections_rails). Specifically, my use case is that I need the specs for my sections to live with the section they belong in, but I also have shared javascript assets and specs that should live in spec/javascripts and app/assets/javascripts (e.g., models, shared views)