Add password command

[wesgarland]

Rather than using send or ctty to enter passwords, niim should have a password (short: pw) command which reads a password from stdin, without echoing it or recording it to history, and then send it to the process under debug, along with a \n, using the same mechanism as send.

If the password command was used with an argument, we could send that argument as the password instead of reading it from stdin.