Traffic plugin should also add incidents to the style #22

The Traffic plugin should also add incidents, such as road closures and detours, to the style, since users don’t generally distinguish between traffic congestion and incidents. Incidents data comes from the Mapbox Incidents source. mapbox/mapbox-navigation-ios#1614 could serve as a starting point, though it’s written in Swift.

Note that the Traffic plugin itself would be obviated by mapbox/mapbox-gl-native#12330.

/cc @fabian-guerra @jmkiley