Support all values of tunnel tag instead of just tunnel=yes

tunnel=* tag has more [documented](http://wiki.openstreetmap.org/wiki/Key:tunnel) values than just yes, for example **culvert** and **puilding_passage** and much more [undocumented ones](http://taginfo.openstreetmap.org/keys/?key=tunnel#values), which still indicate it's a tunnel. The patch makes style support them all instead of just tunnel=yes.