[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[Outdated] Take into account the sources specified in Podfile
Merge pull request #2554 from CocoaPods/outdated-explicit-sources

[Outdated] Take into account the sources specified in Podfile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
[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
[Outdated] Refine pod outdated to show what pod update would do
[Outdated] Use the Analyzer to resolve the unpinned pods
[Outdated] Update for release CocoaPods version
Merge pull request #2501 from CocoaPods/feature/outdated

[Outdated] Refine pod outdated to show what pod update would do
[UserProjectIntegrator] Allows the use of the `GCC_PREPROCESSOR_DEFINITION` flag `${inherited}`  without emitting a warning

Closes https://github.com/CocoaPods/CocoaPods/issues/2577
- fixed configuration specific headers
Modify add_xcode_base_configuration to respect existing base
configurations:
 - Do not change the base configuration when it is already set
 - Notify the user when there is an existing base configuration

Change string quoting from double to single quotes.

Split lines longer than 80 characters into multiple lines.

Quote initial string fragment with single quotes rather than double quotes.
[Changelog] Add link to issue
[CONTRIBUTING] Clarify responsibility more.

We have only so much time in a day, it’s time to take stronger stances on support.
[Pod Installer Source] Ensure `.git` is actually removed

Clarify why .git/ is removed from comparison
[XCConfigIntegrator] Update descriptions to be more verbose

Thanks @alloy!
Merge pull request #2581 from CocoaPods/use_base_configuration

Modify add_xcode_base_configuration to respect existing base configurations
[Downloader] ensure that a downloader failure results in a DownloaderError so the Downloader can catch it

Helpful for https://github.com/CocoaPods/cocoapods-downloader/pull/36