added display-image-on-hover feature, because I wanted it

I was wanting a way to see linked images when I hover the mouse on their link, without clicking through, or using links-to-images to load everything, since that can break the flow of LP posts.

So I made one.

![image](https://cloud.githubusercontent.com/assets/120346/5422760/2c4e7176-8249-11e4-8f7c-ae1ef0b79488.png)

In this example the hover is from the 'a lot fewer' link.