tvOS support

[RomanPodymov]

Hello.
Thank you for MagazineLayout.
In this pull request I added tvOS support.

Details

Added appletvos and appletvsimulator platforms for MagazineLayout.framework.
Added MagazineLayoutExampleAppleTV to test MagazineLayout.framework on tvOS.
Using #if os(iOS) for the iOS-only classes.

Related Issue

Motivation and Context

I think it will be nice to have tvOS support for MagazineLayout.

How Has This Been Tested

I launched MagazineLayoutExampleAppleTV on Apple TV 4K simulator with tvOS 13.4.
I observed the same collection views as in MagazineLayoutExample.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[bryankeller]

Thanks @RomanPodymov ! If you can get the build working, then this seems like a good change! https://travis-ci.com/github/airbnb/MagazineLayout/builds/176573279

[RomanPodymov]

Thanks @RomanPodymov ! If you can get the build working, then this seems like a good change! https://travis-ci.com/github/airbnb/MagazineLayout/builds/176573279

Done.

[bryankeller]

thanks a bunch @RomanPodymov !