Adds ability to disable automatic image dimension application to <img> elements.

We're experiencing issues with our responsive design implementation when images have explicit widths are applied to them. This is also an issue when serving 2x images for high-density displays. Typically, we want images to simply take up only the space that they are provided since they'll likely be in a flexible container.

Normally this works fine, but IE8 in particular is having issues with `max-width: 100%` and `height: auto` on images that have an explicit width.

This PR allows you to disable that functionality.