Layer name aggregator function

[jywarren]

I'm thinking of how we can easily create short-codes for #167 from within an existing map, which capture all the layers currently being shown.

This will help us achieve #252 too -- an embed button!

[jywarren]

I'm thinking this should be something like L.EnvironmentalLayers.getLayerNames() as follow-up to #175

[crisner]

Is this feature something like the one below but to get all the layer names?

leaflet-environmental-layers/src/util/getActiveOverlayNames.js

Lines 1 to 2 in 43a11e3

	L.Control.Layers.include({
	getActiveOverlayNames: function() {