Support external commands beneath brew cask

[rolandwalker]

I'm sure some people may think it is silly to have external-commands-under-external-commands. Rationale:

• We follow Homebrew philosophy where possible.
• There will always be features like "brew cask open" or maybe "brew cask launch" #2246 that only a small number of users are interested in. Why not let them implement that feature for private use? It also gives us a satisfactory and positive way to answer the issue.
• Extensibility in the right hands can lead to unexpected new features, even new projects such as homebrew-cask itself.

[phinze]

Wow, great argument on all three points.

Merging with gusto. 💪