feature-request: support wallet import from hex seed

[grenade]

it would be great to add a --seed arg to compliment the --mnemonic one in wallet import so that people who only have access to the private key seed (rather than the mnemonic used to create it) can import and make use of seed based accounts.

alternatively, replace the --mnemonic arg completely with --seed and just do a regex match to determine if we're looking at a hex seed or a mnemonic...