Support Apple system fonts

[mac-cain13]

Request from @sdevos:

Could we support system fonts, including correct availability. We could use something like https://github.com/mcritz/iosfonts/blob/master/data/iosfonts.json as the source of what fonts are available and then generate corresponding functions in the font struct with availability annotations.

[mac-cain13]

Just found out about https://github.com/Nirma/UIFontComplete. This seems like a good implementation of this feature.

R.swift should be more about your own resources then the Apple standard resources imo. So I'll add a reference to this library instead of building it into R.swift.