Add Voila screengrabber

This application is unusual: it comes in an dmg containing the naked
app rather than a package, but yet it insists on being run directly
from its install location--no symlinks.

I wasn't able to find another example of such a thing in an existing
cask, so I made a stab at doing it manually.

I just checked and it _will_ work via a Mac alias file, but I don't
know how to create one of those short of calling osascript.