QuickLook Plugin Support

OSX [QuickLook plugins](https://developer.apple.com/library/mac/documentation/UserExperience/Conceptual/Quicklook_Programming_Guide/Articles/QLUserExperience.html#//apple_ref/doc/uid/TP40005020-CH3-SW3) are files with the extension `qlgenerator`. They are conventionally installed to installed `~/Library/QuickLook/` (or `/Library/QuickLook`).

As far as `homebrew-cask` is concerned, these are fairly similar to `prefpane`s, so I copied the file structure/naming/testing from the treatment of `prefpane`.

Unlike `prefpanes`, most QuickLook plugin installs would be the *primary* purpose of a `brew cask install` -- without a `.app`. I've included 7 plugins to start with, and they are all like that.

However, some applications ship with optional QuickLook plugins (e.g. `Mathematica.app` offers to install one), so this could also be used to install an application *with* an accompanying `qlgenerator` file.