gifify: Add animated gif command.

Makes animated gifs a bit easier to create.
![example](https://camo.githubusercontent.com/bc0998c24e15d593e310b16a34e8dc091919d453/687474703a2f2f736c65782e6d652f696d6167652f3256317931673354336a33362f6769666966792e6d6f762e676966)

* Still need to manually run XQuartz's pkg file :sob:
* Shamelessly stolen from @SlexAxton

> https://gist.github.com/SlexAxton/4989674
> https://gist.github.com/SlexAxton/4989674#comment-1199058