Description of feature/enhancement

Add "brew cask pin" and "brew cask unpin" commands that work like "brew pin" and " brew unpin" but for casks.

Justification

Now that "brew update" without arguments updates all formulae and all casks, it's nice to have a way of pinning casks that we don't want to have updated. Plus, this brings "brew cask" closer to "brew", which is the direction the projects are heading.

Example use case

E.g, I have robo-3t installed and I'm keeping version 1.2.1 (but 1.4.1 is the latest) because I need to use it with MongoDB versions 2 and 3, and newer versions of robo-3t do not support those. Since there's no way to do "brew cask pin robo-3t", I always need to be sure to call "brew upgrade --ignore-pinned --formula" instead of just "brew upgrade --ignore-pinned" as I was doing before.