Minor: default values for `lon` and `lat` in `df_geojson`.

[layik]

Minor cont. to add characters "lon" and "lat" as default values for function df_geojson.

It is meant to save new users learning about providing column names, and it should kick-start thinking about detecting column names via grep in future.

Could even add z = elevation.

Not quite sure why devtools::document() made some changes to a .cpp file so happy to "reset" that.

[dcooley]

Thanks for the suggestion.

detecting column names via grep in the future.

Given this, it's probably worth copying what I do in mapdeck here - where I auto-detect the lon & lat columns.