MapElementImageProvider does not support implicit positioning

[chris-landclan]

Google's Static Maps API has a feature called "implicit positioning" that sets the map location and zoom level based upon paths and markers provided in the URL. Currently the MapElementImageProvider adds the 'center' and 'zoom' parameters to the URL, preventing implicit positioning from being an option.

See https://developers.google.com/maps/documentation/maps-static/start#ImplicitPositioning

Proposed new feature would add support for implicit positioning when location is not set, provided that there are markers and/or paths, by omitting the center and zoom parameters from the URL.