Add Swift Package Manager support

[aasimk2000]

Adds SPM support. The main change this required was moving the cmark module map to the include directory. This is required by SPM for C languages.

[coveralls]

Coverage remained the same at 93.381% when pulling 0a7235e on aasimk2000:spm into 80d8b2d on iwasrobbed:master.

[iwasrobbed]

Thanks for your help, @aasimk2000 ! I haven't used SPM before so I'll need to dig into this when I get a moment this week to make sure all is 👍 👍

[aasimk2000]

Just want to add that looks like SPM is going to be supported across all Apple platforms in Xcode 11. https://developer.apple.com/videos/play/wwdc2019/408/

[iwasrobbed]

Looks good from my basic understanding of SPM

The only thing missing is the Xcode project file wasn't updated for moving the module map file. Mind ensuring that's reflected (e.g. delete it from the project, drag/drop it back in w/ no target membership)?

[iwasrobbed]

Thanks again @aasimk2000 !