traverse(node, options) {
		this._current = null;
		this._parents = [];
		this._skipped = false;
		this._broken = false;
		this._visitorKeys = options.visitorKeys || vk.KEYS;
		this._enter = options.enter || noop;
		this._leave = options.leave || noop;
		this._traverse(node, null);
	}
Traverse the given AST tree.
@param {ASTNode} node The root node to traverse.
@param {Object} options The option object.
@param {Object} [options.visitorKeys=DEFAULT_VISITOR_KEYS] The keys of each node types to traverse child nodes. Default is `./default-visitor-keys.json`.
@param {Function} [options.enter=noop] The callback function which is called on entering each node.
@param {Function} [options.leave=noop] The callback function which is called on leaving each node.
@returns {void}