Support defs or custom rendering

[Wackymax]

Hi. Firstly, thank you so much for the lib and for keeping it active! I'd like to have a bit more customisability on the SVG that is drawn onto the map. I specifically want to add drop shadows to my lines that I draw. Would it be possible to add some sort of layer/function into this that gets called for me to perform my own rendering logic onto the map image? Or can we maybe add support for passing the SVG defs to the underlying SVG as part of the polyline?