Adding Carthage support

[eliburke]

I was unable to use cmark-gfm-swift without some extra effort. In its current state, if you use normal Carthage methodologies to include this framework (add to Linked Framework and Libraries, post-process using the Copy Carthage Frameworks script) you will get the following error when you try and import the module in a swift file:
cmark.h not found with <angled> include; use ”quotes" instead

A quick fix is to add this to XCode's HEADER_SEARCH_PATHS:
$(SRCROOT)/Carthage/Checkouts/cmark-gfm-swift/**

The correct fix is to modify the modulemap so that cmark.h (and dependencies? unsure) are included in the compiled framework. I believe that the way to do this is to go into the project's Build Phases and edit the Headers section, moving cmark.h from the Private Headers group to the Public Headers group.

[rnystrom]

Nice find. Would love a PR!