HDNode: add optional network flag for importing

This should resolve https://github.com/bitcoinjs/bitcoinjs-lib/pull/321 until `2.0.0`, where hopefully we resolve the current ambiguity between the two interfaces.

> Either we ask users to always provide the network object, or we never do.  Optional arguments are easily forgotten, and therefore [IMHO], dangerous.
Irrelevant since I broke the logic down in a different way anyway: https://github.com/bitcoinjs/bitcoinjs-lib/commit/b6017b0faee731c59909968093b7d58ddbff5cf4