New marks

[earowang]

Maybe:

• [x] mark_mosaic(): https://vega.github.io/vega-lite/examples/rect_mosaic_labelled_with_offset.html
• [ ] mark_parallel(): https://vega.github.io/vega-lite/examples/parallel_coordinate.html
• [ ] mark_geoshape()

[sa-lee]

I've got the mosaic mostly down but need to talk about internals I think.

UPDATE: I've implemented mosaic via a client side transform now, so that basic histogram example works.

[sa-lee]

For mark geo, it looks like we need to convert R spatial objects into topojson. One way is to use https://github.com/ropensci/geojsonio