Symbol map support

[akacarlyann]

Added a demo for a symbol map using icons defined using URLs. @ryanbaumann were you thinking symbol map would make use of the Maki library or is this in the right direction?

To do:

• symbol legend
• custom symbol url property name
• symbol size controls
• Maki symbol support?

Addresses #4

[ryanbaumann]

Thanks for taking a stab at this @akacarlyann - I would suggest we use the map.addImage() function to add symbols to the map style for performance reasons (map.addImage uses the sprite sheet to render images into the GL context) over HTML markers (which render on top of the map GL context). https://docs.mapbox.com/mapbox-gl-js/example/add-image/

Support for default maki icons in symbol maps would be a good batteries-included feature for data visualizations.