Allow {s} in tile urls, and a subdomains array or string

[robbiet480]

I recently added subdomain support to leaflet-tilejson (kartena/leaflet-tilejson#10) for subdomains, as it is hard to implement subdomains with TileJSON in Leaflet. I would like to recommend officially adding {s} as a valid variable in tiles, and an associated subdomains property, which can be provided as an Array or a string, just like Leaflet supports.

[tmcw]

We discussed this and decided not to. It's ending up to be the right decision, since the subdomain hack is fading in importance now that browsers suck less: in the future, there will be one URL always in the host argument.

It's also equivalent to provide multiple domains, versus the {s} hack, if you don't consider Leaflet, and if you consider OpenLayers, the former is more convenient. TileJSON is specifically not a Leaflet configuration language and has other consumers which work very well already - such that a breaking change like this for a single consumer would not be a good move.