Add support for Adobe Air app installations

Adobe Air applications take the form of a .air file that the Adobe Air
runtime then installs and creates a .app -- typically in
/Applications.  This patch provides a new container for those .air
files which the Adobe Air runtime installs to the Caskroom.

Presently, if homebrew-cask doesn't detect the Adobe Air runtime, it
will provide an informative error message and instructions to obtain
it via `brew cask install adobe-air`.  This is obviously not ideal and
the installation process for the runtime should be automated in the
future.