[Validator] Allow the validator to take specific sources

Allows customizable source for `pod spec lint` and `pod lib lint`, with both defaulting to `master`.
Ignore 'InputFile' errors from xcodebuild.
Added a unittest for the InputFile filtering.
[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[Outdated] Take into account the sources specified in Podfile
Merge pull request #2554 from CocoaPods/outdated-explicit-sources

[Outdated] Take into account the sources specified in Podfile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
[Validator] Ensure that the parameter source URLs are used in the Podfile

Didn't work before because `instance_eval` changes the scope of the block