Allow to specify mounted callback on DOM

[lsbardel]

An idea is to specify the mounted callback for a component directly in the dom.

<my-domponent mounted="$callback"></my-component>

and in javascript

vm = d3.view({
    model: {
        $callback: function (cm) {...}
    }
});

In this way one could organise the view model as the application object.

[lsbardel]

not sure why we need this anymore. Too much binding between components.