Issue 1943 allow jscode in options

[hansthen]

Boring but big. The leaflet class hierarchy allows all classes to have optional arguments. These optional arguments can contain JavaScript code or references to other Leaflet elements.

To support this in Folium I replaced (almost) all instances of parse_options in the constructors with this.options | tojavascript in the templates. I left the parse_options in case someone outside of Folium uses it.

[Conengmo]

Boring but big 😄 Kudos for taking this on. It looks good! I have two small comments, plus some suggestions to remove added print statements from tests. Other than that looks good!

[Conengmo]

Let's go! 🚀

[hansthen]

@Conengmo Perhaps you can also have a look at the raster_layers.WmsTileLayer class. I still use {{ this.options|tojson }} there because there is an option that should remain lowercase. The inconsistency bugs me (a bit). I could not think of a way to make it consistent. But perhaps you have suggestions on how we could use tojavascript here?

[Conengmo]

Yes, I noticed that one as well. Can't think of a good, simple solution right now, so maybe what you did is best for now.