Pass any aditional args to L.tileLayer

I think it's good option to pass any additional parameters to L.tileLayer. For example if i want to use mapbox.com maps i have to proved a MapId: `http://{s}.tiles.mapbox.com/v3/MapID/{z}/{x}/{y}.png`
```
L.tileLayer('http://{s}.tiles.mapbox.com/v3/{id}/{z}/{x}/{y}.png', {
    attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, Imagery © <a href="http://mapbox.com">Mapbox</a>',
    maxZoom: 18,
    id: 'examples.map-i86knfo3'
}).addTo(map);
```