text: Title: Ability to draw circles on maps
  Body: The drawing layer can currently draw lines and polygons, but it would be useful to also draw circles (cf [this tweet](https://twitter.com/AlanZucconi/status/930480664234913792)).
  
  The GoogleMaps JavaScript API supports circles (see [docs](https://developers.google.com/maps/documentation/javascript/3.exp/reference#Circle)).
  
  To implement this, I suggest looking at the current implementation for drawing lines on a map -- see [Python side](https://github.com/pbugnion/gmaps/blob/master/gmaps/drawing.py) and [JavaScript side](https://github.com/pbugnion/gmaps/blob/master/js/src/Drawing.js)).