Cocoapods: `pod lib lint` should have --platform flag

[lilyball]

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The pod lib lint command should have a --platform flag to allow me to request that only specific platforms are linted. The idea here is when I'm testing podfile changes that are platform-agnostic, I don't want to wait for it to build 4 different times when building for a single platform is sufficient. It already has the --fail-fast flag but that only makes failure faster, it doesn't make success faster.

[dnkoutso]

This could be a good starter ticket for anyone.

Start here https://github.com/CocoaPods/CocoaPods/blob/master/lib/cocoapods/command/lib/lint.rb and then here https://github.com/CocoaPods/CocoaPods/blob/master/lib/cocoapods/validator.rb

[dnkoutso]

Marking for 1.6.0.

[amorde]

@kballard Implemented as --platforms, allows you to specify a list of platforms like --platforms=ios,watchos. This will be released with CocoaPods 1.6.0

Thanks for the suggestion!