[WIP] Composer support

Implementation for Composer, following #8 
I'm new to Chrome extensions but after building it, it seems to detect the few test packages I've added.

 - [x] Detect composer.json and require/require-dev/conflict etc keys
 - [x] Replace links with repositories from cache
 - [ ] Create task to build full cache
 - [ ] Test

Any thoughts? Do you want me to go ahead or do you want to do it or ..?