`apikey` casing

[jwasilgeo]

Currently this plugin requires the option apiKey to be in camel case, but the rest of esri-leaflet's ecosystem relies on all lowercase apikey.

We should change this plugin to allow for either apiKey (to avoid introducing breaking changes) or apikey usage, while also encouraging the use of the all lowercase style in this repo's README and related documentation at https://github.com/Esri/esri-leaflet-doc/ and https://developers.arcgis.com/esri-leaflet/.

I think at bare minimum the following source code files would be affected:

• https://github.com/Esri/esri-leaflet-vector/blob/master/src/VectorBasemapLayer.js
• https://github.com/Esri/esri-leaflet-vector/blob/master/src/VectorTileLayer.js
• and unit tests https://github.com/Esri/esri-leaflet-vector/tree/master/spec
• README

cc @lillie-bahrami @gavinr

[jwasilgeo]

available on available on https://github.com/Esri/esri-leaflet-vector/releases/tag/v3.0.1