Make it possible to use zoomOffset

[lightoze]

It is a necessary option with some tile providers to use 512px tiles. While it is possible to change tileSize via customOptions, for zoomOffset it does not work because leaflet expects numerical value instead of a string.
Please consider either adding typed customOptions or exposing zoomOffset and zoomOffset.

[octavm]

Fixed and created a pull request to add zoomOffset and tileSize options.