feature: support bip39 entropy source

[tphyahoo]

bip32.org currently supports arbitrary mnemonic (aka passphrase) as core entropy source + 50,000 rounds of hmac-sha256 for stretching, for iiuc a 256 bit seed to feed into bip32.

bip39, another standard for seed generation, is supported definitely by trezor, and, according to

https://www.expresscoin.com/wallets-comparison

alsohive, breadwallet and expresswallet. (This page claims electrum also supports bip39 but this is contradicted by https://electrum.orain.org/wiki/Electrum_seed_phrases)

https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki

This requested feature would add a third option to "derive from" to support bip39:

bip39 standardized wordlist checksummed mnemonic + optional passphrase for 2048 rounds of hmac256 for stretching (as specified by bip39), for a 512 bit seed to feed into bip32.

The bip39 tab should error out if the bip39 seed fails the checksum or does not conform to an approved wordlist.

An additional nice to have would be something to generate bip39 mnemonics.

[tphyahoo]

On second thought, there is significant overlap with existing js tool here:

https://dcpos.github.io/bip39/ (bip32 tab)

maybe it's not worth integrating this feature. (mulling)