osmosis-pgsnapshot: support multipolygons via plepe/osmosis-multipolygon

https://github.com/plepe/osmosis-multipolygon adds support for multipolygons with osmosis-pgsnapshot.
- [x] pgmapcss should detect availability of the `multipolygons` table at compile time
- [x] automatically include additional code in osmosis.py
- [x] naming scheme: 'm1234' (type: mulitpolygon) for multipolygons where tags came from outer ways
- [x] documentation