Add Electrum mnemonic to seed support

[nmarley]

This will allow interoperability with Electrum seeds via:

BTC::Mnemonic.new(words: ...).electrum_seed

The spec works, but I'm not sure where to put the block because it's not one of the "standard test vectors" in BIP39.

[oleganza]

Implementation seems fine by me, but I couldn't quickly find any reference to spec & test vectors. Could you please provide reference in the comment to electrum_seed method and add test vector(s) that are used in other implementations (with a reference to them from the spec source code)?

[nmarley]

The only thing I could find was the Electrum test, so I used the values for that:

https://github.com/spesmilo/electrum/blob/master/lib/tests/test_mnemonic.py#L11-L15

The old mnemonic test isn't really used for modern BIP0032/0039(ish) wallets, so I didn't implement that.