Add "offset" support to CSS styles

By using the scripts from https://github.com/bbecquet/Leaflet.PolylineOffset,
it's very easy to add offset support to the CSS style (f.e. to render cycle lanes, parking lanes, ... next to a road)

Example test query:
```
way [highway=primary] ({{bbox}});
out geom;
{{style:
way
{
  offset: -30;
}
}}
```