Add ability to install casks directly from URL reference

[phinze]

Should be able to do this:

brew cask install http://example.com/path/to/my-cask.rb

It should work just like Homebrew, so maybe we can borrow code from them. This will make it easier to share casks before they're merged.

[chdiza]

I would add that, like homebrew, one should be able to use custom formulae that reside on one's own machine, like so:

cd mycustomcaskage
brew cask install ./foo.rb

More generally, brew cask install file:///blah/blah/blah/foo.rb

[ShaneDelmore]

I want to take a crack at this issue. The immediate question I have is: If a user specifies a cask from an alternate location such as a specific local path or url should I save the cask to a local per user or per computer location for ease of uninstall later and to allow listing of installed casks?

[passcod]

Without categorically addressing the question of whether to do this or not (I vote yea), it should be per computer to keep with Homebrew's general UX.

[passcod]

Implemented in #833.