Add vote for cli command

[jferrant]

Added two commands:

generate-vote: prints a vote signature across a SIP and vote value using the provided key verify-vote: prints whether the message signature verifies against the provided info and public key

e.g. (Note PRIVATE_KEY, PUBLIC_KEY, and SIGNATURE are in hexadecimal format)

stacks-signer generate-vote --sip 10 --vote yes -p [PRIVATE_KEY] stacks-signer verify-vote --sip 10 --vote yes -p [PUBLIC_KEY] -s [SIGNATURE]