[Validator] Allow the validator to take specific sources

Allows customizable source for `pod spec lint` and `pod lib lint`, with both defaulting to `master`.
[Validator] take URLs instead of actual sources
Merge pull request #2566 from CocoaPods/validator-sources

[Validator] Allow the validator to take specific sources
[Outdated] Take into account the sources specified in Podfile
Merge pull request #2554 from CocoaPods/outdated-explicit-sources

[Outdated] Take into account the sources specified in Podfile
[ValidatorSpec] Update 'does filter InputFile errors completely' spec to pass source URLs to Validator
[CocoaPods/CocoaPods#1249] Separate header paths for different targets in .xcconfig files.