Window tagging feature

Hi Eric,

As I am coming from Awesome WM world, I found Spectacle very useful but I missed some kind of window tagging. It made sense to me to implement this feature into the Spectacle.

This feature allows you to tag window with hot key and then go back to the tagged window even if it is on different desktop.

I find myself very productive using it so I wanted share it so I'm wandering if you are interested to integrate this kind of feature into the Spectacle.

This is updated preferences window with default hot keys:
![screen shot 2014-12-23 at 02 06 32](https://cloud.githubusercontent.com/assets/1935686/5533484/43656fba-8a49-11e4-8247-f6dc8a2d37f6.png)