Add Unionizer

This pr addes the unionizer implementation which will take a layers-worth of features and union the 'like' features together into single geometries. There are a bunch of parameters to tweak which are explained in the conf parsing bits. I should add this info to the readme (look for a another commit to this pr). I should also note that this is not tested. I just wanted to get this pr out there for review while I write the tests and documentation. So yeah again, look for more commits.
Linestrings can be zero-length if all points are the same. Is this true of the polygons from mapnik-vector-tiles?