Add ability to check branch/revision-pinned dependencies

[davedelong]

swift-outdated skips dependencies that are pinned to a specific branch or revision, which seems like a sensible default.

I would like the ability to include an option to not skip those dependencies. If I'm joining an existing project, it can be helpful to know how out-of-date those dependencies are. Maybe they were pinned to a dev branch, but the needed changes have since been released, and the dependency can be updated to a normal tag version.