[Proposal] Add `check` command

Ref: https://github.com/Carthage/Carthage/pull/2317

We save and restore `Carthage` directory on CI and run `bootstrap` every time. However, although in most cases it's not needed to build anything, it always checkouts every dependencies which takes relatively long in the process.

So I propose to add `check` command like Ruby Bundler has which checks if the dependencies are satisfied. It's also useful for putting in build phase like CocoaPods does to ensure that dependencis are up to date.