Add Scale option

[jperelli]

see https://wiki.openstreetmap.org/wiki/Static_map_images#Comparison_Matrix

add option to add a scale to the map like google static maps api does it.

• use https://leafletjs.com/reference-1.6.0.html#control-scale
• do it as a boolean option (default to false)

[jperelli]

solved in 707f578081752fb74700d15f3f0111c0e7e2e014, published in npm as v3.5.0