implement new --polygon-desc-file option

 --polygon-desc-file  An osm file (.o5m, .pbf, .osm) with named ways that
                      describe bounding polygons with OSM ways having tags
                      name and mapid

git-svn-id: http://svn.mkgmap.org.uk/splitter/trunk@332 838fabe5-b1a8-4b81-9896-7a84386980e9