Requirements: add cask/download URL DSL

Add `cask` and `download` requirements to `Requirement` and print out `message`s to output them.

This solves two purposes:
- Point more users towards `brew-cask` to resolve requirements
- Provide a way for `brew-test-bot` to automatically install some of these

Happy to tweak the DSL or approach if any of @Homebrew/owners would prefer.