Custom Symbols for scattermapbox

[zach-morris]

This is a request to add functionality for custom markers / symbols in the scattermapbox library.

As it is, the documentation currently says that there are available custom markers available from Maki Icons.

In practice, these icons don't appear to work at all if any theming is present in your application. See here.

The request is to update functionality to allow use of custom marker symbols via svg, similar to Leaflets implementation. This will remove the issue of using the above icons since an SVG could directly be called.