Nicer handling of spatial args

* Most layer functions take `lng, lat` instead of `lat, lng` now
* Layers that can take formulas now also take a data override param
* Lots of ways to assign lng/lat values now: `NULL` to infer from data, formula evaluated against data, or direct. (You can now do `leaflet(data) %>% addCircles()`)
* Don't serialize entire data object to client
Do you need a fallback error message here? (If no lng/lat cars found)