Add `enter` callback as a second .traverse arg

[a-x-]

I suggest making the .traverse method more polymorphic.

is it cool to write:

traverse.traverse(ast, node => isBemjson(node) && bemjsons.push(this.nodeToObject(node)));

insted of:

traverse.traverse(ast, {
    enter: node => isBemjson(node) && bemjsons.push(this.nodeToObject(node))
});