Add example with simple permalink support

This PR is still work in progress. For some reason the panning does not work (mouse cursor is always "finger" when the map is loaded)!?
Since this example is an evolution of the "map-extension" example, it might also be transfered in it.
> One problem right now is that the browser back button will just change the URL to its previous state. 

Is it really a problem? On the contrary I would find it interesting to be able to navigate in the browser history to revert/restore changes done by the user on the map (zooming, panning, layer adding, etc.).

Especially with your debounce tool that avoids having to many intermediate panning states.