update node.extend to avoid vulnerability

[danielsap]

Could you please update node.flow and rmdir to use node.extend >=1.1.7, <2.0.0 or >2.0.1 so as to avoid vulnerability complaint? node.extend v1.0.8 and v2.0.0 has Prototype Pollution vulnerability. We rely on your rmdir which currently cannot pass security audit by npm. Thanks!