Add Carthage support

[jmkiley]

Currently the Mapbox Maps Plugins can be installed via CocoaPods or manually. Carthage support should also be offered.

[1ec5]

#9 introduces subspecs. Per mapbox/mapbox-navigation-ios#108, it probably won’t be possible to use subspecs while supporting Carthage.

[jmkiley]

Good to know, thanks! We could go back to individual podspecs, then offer bundled podspec later that installs all plugins. 🤔 Would multiple podspecs pose an issue once we have more plugins?

[1ec5]

Yeah, I could see multiple podspecs being a royal pain to maintain. Even just pushing two podspecs for the navigation SDK project to CocoaPods trunk is a bit error-prone. (I always forget to run the slow pod spec update process between each podspec push.)

This plugins project would be the perfect fit for subspecs, but the root of the problem is that it’s incompatible with Xcode’s project model. It might still be possible to hack around the problem, as long as we’re really judicious about imports. That might mean eliminating PluginKit entirely.