Restrict possible min/max zoom values

[davidbgk]

Fix #1318

[yohanboniface]

The absolute max zoom value is a bit tricky to define once and for all. The tilelayer is at the end what will usually define this value for a given map.

20 seems legit, but some custom use cases may want a bit more. Also, we need to also limit the options.overlay.min/maxZoom. So I'd suggest having a map.options.absoluteMaxZoom hard coded, that could be then used where needed and will prepare in case we need to expose that option to users at some point.