RFE: Implement SEP-0005

[kfiresmith]

Implement SEP-0005 for 24-word wordlists

I believe that implementing https://github.com/stellar/stellar-protocol/blob/master/ecosystem/sep-0005.md would be very good for Firefly wallet security and recover-ability.

I would particularly like the option for 24-word wordlists that are additionally protected by a passphrase (see test case 4).

A Javascript implementation can be examined here:
https://github.com/chatch/stellar-hd-wallet

[manran]

I think it will be a good feature for firefly. @MaZhaoYong