Don't add webkit prefixes when a data-uri is set

I've made a "CSS shorthand filters to SVG" processor here: https://github.com/iamvdo/pleeease-filters and integrated in Pleeease.
WebKit browsers don't support SVG filters through data URI, so prefixes can be removed safely in this case.
Merge pull request #273 from iamvdo/patch-1

Don't add webkit prefixes when a data-uri is set