Add CLI argument parsing to sign-message.py example.

[juestr]

This adds command line parsing of message and key to the sign-message.py example:

$ ./sign-message.py -h                                                  (sign-message-cli|✔)
usage: sign-message.py [-h] [-v] -k KEY -m MSG

Sign a message with a private key.

optional arguments:
  -h, --help         show this help message and exit
  -v, --verbose      verbose output
  -k KEY, --key KEY  private key in base58 encoding
  -m MSG, --msg MSG  message to sign

Security warning: arguments may be visible to other users on the same host.

$ ./sign-message.py -m 'hello world' -k 'L4vB5fomsK8L95wQ7GFzvErYGht49JsCPJyJMHpB4xGM6xgi2jvG'
H5fuPQBm4Vmb5q19cbxgkH769SgSkotl9I+qGgKF3ytyE8L/rZgq3c1aYPY4wIAVNlzJ++T4+6WZlKO5diIukyY=

$ ./sign-message.py -v -m 'hello world' -k 'L4vB5fomsK8L95wQ7GFzvErYGht49JsCPJyJMHpB4xGM6xgi2jvG'
Address: 1F26pNMrywyZJdr22jErtKcjF8R3Ttt55G
Message: hello world
Signature: b'H6rxzfQC1l65vzDaGyqTkO5r7QQuwXfEocem7YYP6+3vdwUrs0fyJSfodeKyf61Z1V5Bqfstpt68aSty3rZgq7M='
Verified: True

To verify using bitcoin core:

`bitcoin-cli verifymessage 1F26pNMrywyZJdr22jErtKcjF8R3Ttt55G 'H6rxzfQC1l65vzDaGyqTkO5r7QQuwXfEocem7YYP6+3vdwUrs0fyJSfodeKyf61Z1V5Bqfstpt68aSty3rZgq7M=' 'hello world'`