Add fields_to_exclude option to map.render()

[ljbade]

Part of mapbox/mapnik-vector-tile#78

@springmeyer Am I doing this part right?

[springmeyer]

looking on track. you can pass options down to node-Mapnik via tilelive-bridge. so you could start testing locally by editing tilelive-bridge index.js manually. after that we could start thinking through how to pass options down from the studio UI or project file.

[ljbade]

Hmm I need to figure out more of the JS stuff. All new to me.

[springmeyer]

@flippmoke and I are pushing hard on various improvements to the vtile API. This would be a great improvement to make once other stuff has landed. So, I'm going to assign to the v3.4.0 milestone that will tackle after other big pushes are in place: #421