Prepare for Homebrew Cask to be imported.

[MikeMcQuaid]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run brew tests with your changes locally?

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• ignore Cask's files in readall (for now, there's an intentional syntax error that will need fixed)
• run Cask's tests if they exist
• don't check Cask's files in coverage reports (for now)

After merging this should make #725 💚, have non-terrible coverage reported and run casks tests.

CC @AnastasiaSulyagina