Add support for Carthage

[Den-Ree]

[REQUIRED] Step 1: Describe your environment

• Xcode version: 10.1
• Pring version: 0.15.0

[REQUIRED] Step 3: Describe the problem

If use the framework through CocoaPods, it takes too much time on our CI (Bitrise.io) for deploying, so we can't deploy the build.
We have tried to move in a separate framework, but than we had the next problem: when Pring tries to call Firestore, it returns warning that the FirebaseApp is not configured.

[Den-Ree]

@1amageek Hi, will you review the PR with support for Carthage, if someone from our team implements it?

[1amageek]

Yes of course