Implement `__geo_interface__`

[mvexel]

Instead of the homegrown GeoJSON implementation, we should consider implementing __geo_interface__ to enable interoperability with widely used geospatial Python modules such as shapely, geojson and fiona.

I haven't looked into this much yet, but I am considering it for the osmdiff project that I am currently refactoring.

/cc @sgillies who came up with __geo_interface__ in the first place.

[mvexel]

See also

• Multipolygons are breaking geojson convertion #135
• Support relation on geojson #48
• PR Replace homegrown converter with osm2geojson #140