Adding geojson callback to be able to do some post processing

Hi !

I have discovered this great project to integrate leaflet in my angular project, and I have some question about geojson part.
I want to fit map bounds according to geojson data, and I don't know how to do it properly.
If you look at my PR, an ugly callback has been added to be able to define, on geojson options, a `postLoadCallback` : 

```
geojson: {
    data: ...,
    filter: ...,
    postLoadCallback: function(map, feature) {
          // With this call, map will always covert geojson data area
          map.fitBounds(feature.getBounds());
    }
```

Does somebody know how to do it properly with angular/leaflet directive philosophy ?