Add Black Screen.app.

[vlad-shatskyi]

Adding a new cask

• Checked there aren’t open pull requests for the same cask.
• Checked there aren’t closed issues where that cask was already refused.
• When naming the cask, followed the token reference.
• Commit message includes cask’s name.
• brew cask audit --download {{cask_file}} is error-free.
• brew cask style --fix {{cask_file}} left no offenses.
• brew cask install {{cask_file}} worked successfully.
• brew cask uninstall {{cask_file}} worked successfully.

[vlad-shatskyi]

The repository URL, for your convenience. https://github.com/shockone/black-screen

[worst-developer]

@ShockOne Add brew command to readme

[vlad-shatskyi]

We should be discussing it here, @yarossia railsware/upterm#350. I've already changed README in a feature branch. Will be merged as soon as I finish setting up automatic updates.