Add support for "mapnik-geometry-type" in filters

This is a simple implementation of support for ``mapnik-geometry-type`` use in filters.
See https://github.com/mapbox/carto/issues/180

I'm probably missing something about the use of variables (ie: should we allow a variable to _expand_ to "mapnik-gemetry-type" ? I suppose that part isn't implemented).

Also, this implementation doesn' t use any support of mapnik-reference