turn this proof of concept into a plugin

[jgravois]

resolves #2. now you can just call:

// any of: DarkGray, Gray, Hybrid, Navigation, Streets, StreetsNight, StreetsRelief, Topographic 
L.esri.Vector.basemap('DarkGray');

• bump urls to leverage latest vector basemap styles
• rewrite proof of concept to leverage an ES6 bundle instead
• extend MapBoxGL class
• add error handling for style requests
• pass dummy token
• don't worry about auth (because our basemaps don't require it)
• duplicate attribution code from basemaps (work on moving to utils later)
• figure out why its not possible to call addTo(map) immediately after instantiating the basemap. (right now the async request to fetch style json triggered during object instantiation is a problem.)
• bundle the fork of mapbox-gl-leaflet dependency internally
  composite raster overlays in relief and hybrid basemaps are dropped entirely.
• bundle the fork mapbox-gl dependency (required adding a jsnext:main field
  to do:

after this lands i can:

• change repo name to esri-leaflet-vector-basemap
• improve the attribution logic to make sure it disappears if/when the layer is removed
• try and figure out how to bundle the minimal esri-leaflet dependencies internally

[jgravois]

just need to wait for onAdd to fire async style request...

[jgravois]

i finally have something i'm fairly happy with here.

since we are dealing with dependencies that are forks of other projects, it seems like the best course of action is to just bundle them right in this plugin. they aren't written in ES6, so i had some fun learning how to tweak the rollup config to bundle via 'main'.

an @Esri/leaflet-contributors code review (probably @patrickarlt) would be super helpful.
if everything looks good i'd be ready to tag a 1.0.0 release.

[jgravois]

@manders i've pushed up a few commits so that the plugin will just work after a fresh npm install. let me know if you have any trouble getting it running yourself.

[patrickarlt]

👍 🎆

Looks like zoom animations are flickery again because of mapbox/mapbox-gl-leaflet#47 though so that makes me sad :(