Added support for <image> tags with xlink:href and src linking

During development we encountered a scenario were <image> tags (both for those using 'xlink:href', as well as 'src' ) were not being versioned, 

At first we tried using the 'patterns' attribute in options, however that meant overriding the default one.

Thus we've opted for adding support for these two patterns with the ones which already exist.
This is one of [SVG fallback techniques](http://css-tricks.com/svg-fallbacks/).