Add memo encoding helper
function/helper to encrypt memo, consider using public-key encryption as oppose to aes. There are some pros and cons, @netuoso suggested public key encryption.
pros: it doesn't require private key of sender and only public key of receiver should be sufficient cons: only receiver will be able to decrypt the message with their private key, sender won't be able to decrypt in future I believe.
AES implementation hivejs uses allows sender to decrypt as well, but it requires private key of sender to encrypt.