Fixes #18

Because I don't know better just for urls now...maybe you know a more general way to omit string values (as url() don't have to be in quotes necessary...e.g. url(../path/to/file.ext) is as valid as url("../path/to/file.ext") )? =)...but there could also be something like content: "superbox"; which would probably also be affected by vendorprefixing...

Fixes: https://github.com/oscarotero/stylecow/issues/18

EDIT: Just tried, content: "superbox"; doesn't get prefixed nor does content: "boxcontent", but content: "box-content" does...^^...seems quite specific =)