Support url() with quotes in unknown directives

[nex3]

Currently, libsass fails to parse a url()-style string with quotes when it appears in an unknown directive. For example:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

produces:

Error: Invalid CSS after '...pace url("foo");': expected "{", was ""
        on line 1 of stdin
>> @namespace url("foo");
   --------------------^

See sass/sass#2188