prefer place tag for address details

This is an attempt to fix problems with the labels of boundaries in the address details (a) when unusual admin levels are used (e.g. #51) or (b) when boundaries span multiple admin levels (e.g. German city-counties like [Frankfurt](http://www.openstreetmap.org/relation/62400)). The idea is that if a place tag is found in extratags then this is preferred for determining the label for the address details.

There are currently around 54k boundaries with place tags, so this change might have a rather noticeable effect on the output.