Add a 'text-backup' tool

[za3k]

Add a separate command line tool text-backup which generates a text-mode backup instead of using QR. It should be keyed back in by hand.

Ideas:

• Include checksums and ECC per-line
• Can we make it "readable" if the original is text, so that you don't have to key in some text to read other text?
• Try to make it easy to type? BIP39 wordlist, PGP word list, etc may be a good option but then need the wordlist to decode.
• Generate .txt by default with a .pdf option

This will require factoring out the "QR" backup, restore, and test process from the rest of the logic. Could be done before or after adding erasure coding.

[za3k]

won't fix. would be a different project, although a good one