Multiple updates to WindowResizer including custom size

First off, thanks for making a very useful and clean Chrome extension. 

My fork came about because the `chrome.windows.update` method on Chrome for Mac sets the outer bounds of the window size, but I needed to set the inner viewable area, not the outer window. I set about making some edits and ended up adding several features:

 - Now sets the inner window size (default). 
 - You can change which style of resize is used via radio buttons.
 - Custom size inputs implemented.

Known Issues: 

 - The settings are not saved, and the UI returns to defaults every time it opens. I thought about using `localStorage` as a way to save and restore settings, but got lazy.
 - My added code is functional, but inelegant. Apologies for making a mess.
 - I'm on a Mac, so this is untested on Chrome for Windows. Actually, it's pretty much untested outside of two Macs running current Chrome builds.

Thanks again.