Add layers control options support

[pomahtuk]

Example call

$scope.leaflet = 
  center:
    lat: 59.95
    lng: 30.3
    zoom: 10
  layers:
    options:
      collapsed: false
    baselayers:
      osm:
        name: 'OpenStreetMap'
        url: 'http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png'
        type: 'xyz'

[tombatossals]

Thanks, merged.

I was thinking about making the controls independent of the layers, and manage them from a directive property. Like this:

<leaflet layers="layers" controls="controls"></leaflet>

So we can have the option of show/hide the controls that we want from this object:

$scope.controls = {
    layers: {
        active: true,
        collapsed: false,
    },
    locate: {
        active: true
    }
}

But that will be in the future, your commit makes sense in the actual code.

Thanks!