Add electrum mnemonic support

[thecodefactory]

Implement support for electrum mnemonic and seeds with additional bx commands electrum-to-seed and electrum-new.

[skaht]

It is worth noting that Electrum seeds are fully invertible unlike BIP 39 technology.

[thecodefactory]

@skaht @evoskuil I'm still trying to determine if we need to support electrum 1.x mnemonics (which libbitcoin implemented in the past but deprecated in v2 and removed in v3 I believe), or the newer electrum 2.x mnemonics, which are somewhat similar to BIP39, but include a version prefix.

If the former, we can just revive the previously existing electrum mnemonic code, dust it off and call it a day. If the latter, I spent some time working on that and have working code in a private libbitcoin branch, but need some feedback on how best to expose it from bx. Either way, I think we'll be covered, but we need to get to the bottom of what's really needed here. Perhaps we need to support both versions? I don't have any clue how much use either version may have in the wild.

[evoskuil]

I see no reason to support older electrum mnemonics as IIRC electrum can upgrade them.

[evoskuil]

WRT seeding, the implementation should only accept a seed as a parameter and should follow the conventions of existing commands that use seeds.

[thecodefactory]

Update on this (as someone asked on the IRC channel):

The draft implementation I have is here:

https://github.com/thecodefactory/libbitcoin/tree/electrum-mnemonic

I need to do a once over, as it's been a while since I looked at this. I also need to clean up the draft explorer support. Pull requests coming sometime after that.