Convert data to 'GeoJSON' or 'TopoJSON' from various R
classes, including vectors, lists, data frames, shape files, and
spatial classes. 'geojsonio' does not aim to replace packages like
'sp', 'rgdal', 'rgeos', but rather aims to be a high level client to
simplify conversions of data from and to 'GeoJSON' and 'TopoJSON'.