Replace all the ?format= stuff with middleware handlers

[thisisaaronland]

Let's assume we're talking about PIP operations.

• Everything should start by being run through /api/point-in-polygon.
• If there is a transformation it should happen in a separate middleware handler.
• We can test this idea by re-enabling geojson output as something like /api/point-in-polygon/geojson.
• If that works we'll need to do the same for ?format=properties

We'll need to update the /api/point-in-polygon handler to accept an optional "next" handler.

[thisisaaronland]

This is no longer relevant following the use of whosonfirst/go-whosonfirst-spr-geojson.