Support app extension targets through cocoapods

[DarthMike]

Continuing stuff from #301:
When importing Snowplow through cocoapods, and using it in extension and app, the library is compiled once, which causes a problem, as we need to compile twice with different macros (SNOWPLOW_APP_EXTENSIONS in this case) to not use unavailable API in extension.

Cocoapods team discusses this can be supported by using a podspec only for extensions, defining the macro in the pod itself, so user of library doesn't have to do it.
CocoaPods/CocoaPods#5373

It would be great if we could support this here

[AlexBenny]

Closing because it has been with no activity for a long time.
Please, feel free to reopen or comment this issue, whether it's of your interest or you want to contribute on this.