[Validator] Allow the validator to take specific sources

Allows customizable source for `pod spec lint` and `pod lib lint`, with both defaulting to `master`.

@orta, I'd like your take on the CLI message for those options.

@alloy right now this pulls in all repos for `pod repo push`. Not sure if that's the behavior we want.
You could just pass around the URLs, so the sources will be always created during the installation if needed. Performing an action only in one place makes the code more predictable and easier to debug.