custom colors for default markers

[andrewharvey]

closes #6412

Launch Checklist

• briefly describe the changes in this PR

Allows users to customize the color of the default Marker (option 1 at #6412 (comment))

• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page

I haven't used the style-spec Color type as it outputs rgba and SVG doesn't accept alpha in the color, https://www.w3.org/TR/SVG/types.html#DataTypeColor

[jfirebaugh]

Great!

[qvs5010]

Hi

Outstanding !!!!, are the changes live, can we use, or must we wait for a version upgrade?

v0.44.1/mapbox-gl.js

    var r = Math.round(Math.random() * 255);
    var g = Math.round(Math.random() * 255);
    var b = Math.round(Math.random() * 255);

    var marker = new mapboxgl.Marker({
        color: `rgb(${r}, ${g}, ${b})`
    })

[qvs5010]

Oh, I basically answered my self... lol, "not yet"