[Improvement] Check for invalid master key per BIP-32 #144

I know this is a BIP-39 repo and not a BIP-32 repo, but it would be nice to add a flag to check for BIP-32's master key generation spec, which states:

Use parse256(IL) as master secret key, and IR as master chain code. In case parse256(IL) is 0 or parse256(IL) ≥ n, the master key is invalid.

If the master key is indeed invalid, a ValueError can be thrown or perhaps extra entropy can be added until a valid master key is generated. If this seems like something someone would want, I can push a PR.