Support change of archive passphrase

[tasket]

Allow user to change the passphrase used for a Wyng archive. This would likely involve storing the actual key in what is now the '.salt' file, encrypted by the chosen passphrase + salt.

There should also be a command/parameter structure that supports such changes to an archive, such as property --passphrase= or similar.

[tasket]

@rapenne-s I might try adding passphrase changing soon, but it would be considered an experimental branch of the Wyng archive format (at least until the details are settled and I'm certain of correct implementation). If you're interested in testing that, let me know.

[kennethrrosen]

Happy to test as well @tasket