Added custom sized images to the media manager dropdown selection

I needed to be able to select my custom sized images on the post editor screen in the media manager and it was a really simple addon so I added it and sent this pull request.  Hopefully it will be accepted to the Roots project, thank you

The image below shows what it looks like with my custom sized images...

![npm7sbf](https://cloud.githubusercontent.com/assets/186818/3621984/38673f90-0e24-11e4-98c0-bee02cc5d3b2.png)