Introduce HEREv3 (using apiKey instead of app_id and app_code)

[mcguffin]

Use Tile API-URL and apiKey as described in latest HERE docs: https://developer.here.com/documentation/map-tile/dev_guide/topics/quick-start-map-tile.html
Will fix #342

[brunob]

@jieter any thoughts ?

[jieter]

Yes, the readme links to the preview, using [Available HEREv3 layers](http://leaflet-extras.github.io/leaflet-providers/preview/#filter=HEREv3), but 'HERE' is completely ignored from preview:

leaflet-providers/preview/preview.js

Line 81 in 52bfb50

if (providerName.startsWith('HERE') || providerName.startsWith('OpenWeatherMap') || providerName.startsWith('MapBox')) {

The reason being that we kept exceeding the free plan, even with a single layer in the preview list.

I'd say there is limited value in adding these providers we cannot reliably show previews for, as they are not really discoverable.

[brunob]

@jieter nice catch maybe it could be better if @mcguffin remove the mention & link to preview for HERE in the readme ?

[brunob]

LGTM

[jieter]

Merged, thanks @mcguffin

[stell]

Can someone tag 1.9.2 with this update? Thanks!

[brunob]

@stell no hurry, we were waiting to merge others PR, now that it's done, we will shortly publish a new tag when our free time will allow it :)