add new option custom_node_render to support vary rendering logic

[hizzgdev]

Add new option custom_node_render to support vary rendering logic.

Now, It's possible to customize your rendering logic for specific node.

let options = {
    ...
    view: {
        custom_node_render: function (jm, element, node) {
            // any customized logic
            let updatedTopic = '<img src="">'+node.topic;
            element.innerHTML = updatedTopic;
            // return true to tell jsMind the node has been rendered
            return true;
        }
    }
}