Add linter to detect input asking for password

[mschwager]

Bad:

input("Password: ")

Good:

getpass.getpass("Password: ")

See getpass.

We can probably just look for input calls that contain the string literal 'password'.

[mschwager]

Note this may be somewhat moot since we're already searching for all input usage in DUO108. However, six.moves.input and input in Python > 3.0 are whitelisted, so probably still worth it.