Add class_name argument to more map elements

[pixelsapphire]

LeafletJS reference mentions a className option for several classes, such as Path (https://leafletjs.com/reference.html#path). The option is used to give elements custom CSS classes. It is implemented in Folium as the class_name keyword argument, but it is currently only available for DivIcon and GeoJsonPopup.
I would like to be able to give custom classes to PolyLine elements (and other BaseMultiLocation elements) via the class_name keyword argument.
Currently, I'm abusing the fill_color argument (and setting fill_opacity=0) as a place to store the classes to be later accessed and properly added by a JS script, but this solution is far from perfect, to put it mildly.

[Conengmo]

This is a good point. Classes like PolyLine accept **kwargs, but then this gets stripped in the path_options function.

I don't think we should add class_name as explicit keyword arguments, but we should allow users to pass it through **kwargs.

We should add className as a valid option in the **kwargs of path_options. Should be easy to add and test for. PR's are welcome!