Allow custom mapbox maps for base reference maps in browse tool

[neilgallen]

First: Including mapbox in 2019.4 is GREAT!

That said, one of the great features of mapbox is the ability to customize your own map layers. 

It would be great if this was an option to use a custom map as the alteryx reference map similar to what is available within tableau. We in our organization have several custom map styles built with mapbox and I'd love to use them as a reference in the report map tool, or any other scenario where maps become useful.