Augmented-Diff Support

[tyrasd]

(Overpass API now allows stuff like custom filtered augmented diffs, nice!) See drolbr/Overpass-API#103

As a first idea, I'd probably like to add a second parser parallel to osmtogeojson, that produces GeoJSON for augmented diffs. That would allow straight forward integration in the current pipeline: for example MapCSS, exports, etc.

The code used by achavi can probably be reused partly (but the lib/parser currently only talks directly to some OpenLayers APIs, as far as I can see…). Maybe some consolidation with osmtogeojson would make sense, too. Also related: tyrasd/osmtogeojson#21