Extend the adminizer to handle points

@zerebubuth 
Yeah, the [WKT parser](https://github.com/mapnik/mapnik/blob/master/include/mapnik/wkt/wkt_grammar_impl.hpp#L67) in the Mapnik source would seem to suggest it's a single entry with `SEG_MOVETO` as the command, and that multi-points are just multiple single-entry paths. However, much of the code seems not to mind about (or, at least, not to check) the command on the first vertex.