Support reversed zoom coordinates

[Majiir]

This pull adds a zoomReverse option to the TileLayer class for reversing the order of zoom levels. The option is disabled by default.

[msingleton]

What's the use case here? Seems like this might be a rare enough request that it's better to be handled by your application rather than in the library

[Majiir]

My application generates tiles with the zoom numbering reversed: level 4 is more zoomed-out than level 3. The world is infinite, so zoom level zero corresponds to 1 pixel = 1 meter, and higher/lower levels branch from there. As far as I can tell, other applications similar to mine use the same zoom indexing.

A more flexible method might involve a snap-in function for converting zoom/coordinates to a tile filename.

[JasonSanford]

Yeah, I'm with @msingleton. It would probably be best to override L.TileLayer for your use case. Or maybe reversed zoom number is more common than I'm aware of?

[Majiir]

This pull looks a little hackish, but I think that's more a reflection on how zoom numbering is handled in general; Leaflet isn't particularly elegant at handling either reversed or offset numbering schemes. At least in my case, there's an entire application domain where the numbering is both reversed and offset.

[mourner]

I agree that zoom number mapping in Leaflet should be done through a separate function which can be overriden, and since the reverseZoom option doesn't add much to the code anyway, it's useful enough to be merged. Thanks!