Allow multiple 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)