vector layers: allow both snake_case and lowerCamelCase options

[Conengmo]

Closes #1452

In folium we use PEP8, which means function arguments and variables are in snake_case notation. This can be confusing when users look up path options from Leaflet for vector type layers, which use lowerCamelCase notation. We refer to these options in multiple places, for example here:

folium/folium/vector_layers.py

Lines 152 to 153 in 5b079fb

	Other valid (possibly inherited) options. See:
	https://leafletjs.com/reference.html#polyline

Address this by allowing both snake_case as lowerCamelCase notation for vector path options. Implement this by converting the function arguments directly to lowerCamelCase within the path_options function. This way, both notation styles are merged into one: lowerCamelCase. Which is also the notation used in the output of this function.

Note that the parse_options function we use in other places already has this dual behavior currently.

Since we are sticking with PEP8 we don't have to broadcast this in every piece of documentation, but it should prevent more issue reports on this topic.