Skip data URIs when making URLs absolute

Currently, applyPrefix will convert URIs such as
url(data:image/svg+xml;base64,PD94bWwgdmVyc2lv) to absolute urls. This
patch uses (?!data:) to fail the RegEx match where the URI starts with
"data:"
Merge pull request #16 from syslabcom/master

Skip data URIs when making URLs absolute