feature: Enable app Podfile

From @DickSmith on June 10, 2018 21:59

Sometimes a project just needs to add a Podfile without any JS/TS, but a full plugin must be created:

1. https://github.com/tjvantoll/nativescript-IQKeyboardManager (Until recently was only a podfile)
2. https://github.com/Essent/nativescript-disable-bitcode (just a postinstall to strip all bitcode from a project).

Additionally, this is where a project should be able to set its platform version level, as well. (and plugin Podfiles should not).

In many ways this is similar to many of the settings and package management actions that can be taken in app.gradle (dependencies not requiring JS, package overrides, and min platform).

Would likely just come from App_Resources like similar files.

Copied from original issue: NativeScript/ios-runtime#939

[DickSmith]

@NickIliev https://nstudio.io/blog/nativescript-app-specific-podfile/ Link screwed up before, sorry.

[Fatme]

Acceptance criteria:

• Ensure the Podfile from App_Resources is correctly merged in platforms folder
• Ensure the application is successfully run on device when Podfile in App_Resources
• Ensure the content from project’s podfile is removed when Podfile in App_Resources is removed