Accept custom color palettes in create_color_stops()

[ryanbaumann]

Problem

Currently, the create_color_stops function accepts a string that defines a color palette from ColorBrewer. This function should also allow the user to pass a custom color palette as a list of color strings. The documentation claims this is possible today, but it is not yet implemented.

Solution

Make a PR to add support for passing a custom list of colors to create_color_stops -> https://github.com/mapbox/mapboxgl-jupyter/blob/master/mapboxgl/utils.py#L109

cc @pveugen

[ppessi]

Should it contain a check for if the list is valid?

[ppessi]

I don't know how to read, I thought that the reference by ryanbaumann was a reference from another issue and not a commit :D Oh well

[ryanbaumann]

I like your implementation, thanks for the PR @ppessi! We'll use your suggested color checking method in the existing PR, it's cleaner and doesn't require another library dependency -> 993b719