Adminizer post-processor

The adminizer allows you to select (from a Mapnik datasource) some polygons to use to set or update parameters on the features which pass through the "izer" post-processing steps. The original intent of this is to add admin polygon information such as state and country, which can then be used to control the styling of those elements. However, there are many more uses, such as styling urban and rural regions differently.
Well spotted, thanks! I've added a test case and fixed in 93e7157.