@require mixins in modules for better dependency management

[GCheung55]

Most of the modules have a dependency on a module in the mixins directory. But those dependencies have to be manually declared.

For example, if I wanted to use bootstrap/carousel, I'd have to write my stylus code like this:

@require "bootstrap/mixins/gradients"
@require "bootstrap/carousel"

It would be very helpful if the dependencies are declared/managed within the module that is being used. With the carousel example above, it just means @require "bootstrap/mixins/gradients" would be declared inside bootstrap/carousel.

[jangarcia]

+1

[maxmx]

To be considered for the BS4 port.