Mapbox GL support

Hi, I have happily used Mapbox and Omnivore together on my personal site for a couple years. They make a fantastic pair and I was hoping to continue using Omnivore while switching to Mapbox GL.

Since Omnivore is advertised as a glue library, I assumed its GeoJSON output can be consumed by MapboxGL as well, but I can't quite figure out the incantation to get it working.

I see some hardcoded references to `L.geoJson()` in Omnivore when no additional args are given to the parsing function. Since `layer` is something that can be passed in, I thought maybe I could override but nothing I tried seems to be working. Code-wise, I'm basically using the Omnivore example found here: https://www.mapbox.com/mapbox.js/example/v1.0.0/omnivore-kml-tooltip/

Start by reviewing the parser APIs and the hardcoded L.geoJson() references mentioned in the issue, then compare them with the linked Mapbox GL example. Determine whether the existing layer argument can support Mapbox GL and define a reproducible integration case as the completion criterion.

Written by the indexing model from the issue text.