GLTFLoader: Add loadNode hook

[takahirox]

Description

This PR adds loadNode hook to GLTFParser. /cc @donmccurdy

Use case:

This hook is needed for glTF LOD extensions (MSFT_lod or EXT_node_lod)

// LOD extension plugin
loadNode(nodeIndex) {
    // Some set up here
    ...
    //

    const lod = new LOD();
    const pending = [];
    pending.push(parser.loadNode(nodeIndex));
    for (let i = 0; i < extensionDef.lods.length; i++) {
        pending.push(parser.getDependency('node', extensionDef.lods[i].node));
    }
    return Promise.all(pending).then(nodes => {
        for (let i = 0; i < nodes.length; i++) {
            lod.addLevel(node, coverages[i]);
        }
        return lod;
    });
}