add support of custom controls

# Usage
## Partial
``` html
<leaflet markers="markers" center="center" events="events" custom-controls="myCustomControls">
</leaflet>
```
## Controller logic
```javascript
//setup custom controls
    $scope.myCustomControls = [];

    var MyControl = L.Control.extend({
        options: {
            position: 'bottomleft'
        },

        onAdd: function (map) {
            var className = 'leaflet-control-my-location',
                container = L.DomUtil.create('div', className + ' leaflet-bar');

            // add here MyControl view & logic

            return container;
        }
    });

    $scope.myCustomControls.push(MyControl);
```

more about custom controls http://leafletjs.com/reference.html#icontrol