`Formatter`: Add `shape` option

[clarisma]

shape would allow user to specify a function that supplies the geometry for a given feature; this could be used to apply various transformations, e.g.:

features.geojson(shape=lambda f: f.shape.simplify(100))

(See clarisma/gol-tool#80)

• shape must be a function that takes a Feature and returns a Geometry

• If shape function is provided, mercator is ignored; coordinates are output as-is (only precision applies)