Allow removing individual geometry #501
Description
The APIs around adding polylines, polygons, and markers are currently unbalanced in that you add individual geometries but then only have the option to clear all of them. We should add methods to remove individual polylines/polygons/markers and update the current methods which remove all polylines/polygons/markers so that the apis for each look like this:
//Currently exists
`MapzenMap#addPolyline(Polyline)`
//Add this
`MapzenMap#removePolyline(Polyline)`
//Currently is `MapzenMap#removePolyline()`
`MapzenMap#removePolylines()`