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") )? =)...
A bit better

can catch strings in double or single quotes now!
Merge pull request #21 from exside/patch-2

Fixes #18