Extend the Cartfile

Carthage is simple. So is this issue trying to be (although the first word in the title gives it away already).

Looking through the issues around here people seem to want to specify dependencies:
- per scheme
- per platform

Possible solutions:
- yaml
- ogdl
- prefixes and/or suffixes

While the discussions are long on this subject, I suggest we keep this short and focused, to decide in general whether we want this or not. I'll try to update the above lists if new ideas come up.