Added option which will try "keyboard-interactive" user authentication

[mashaalmemon]

Added ability to handle "keyboard-interactive" user authentication if primary authentication fails.

My use case for this feature is discussed in #31 . This works perfectly with user authentication when SSH server setup to use public key and multi-factor authentication. To try this out you can setup an SSH host on AWS as discussed in this article: https://www.middlewareinventory.com/blog/aws-mfa-ssh-ec2-setup/ and ssh to it.

Options for such a setup would be as simple as:

export const sshConfig:Options = {
    endHost: '<ssh_host>,
    username: '<ssh_user>',
    privateKey: fs.readFileSync('<private_key>'),
    tryKeyboardInteractive:true
}

If "keyboard-interactive" authentication is initiated by the server, prompts passed in by the server will be shown, and the library will wait for keyboard input from the user before proceeding.

Note that if is set to true but "keyboard-interactive" authentication is not initiated by the server, the option will do nothing.

The README.md file has been updated to include a description of the new tryKeyboardInteractive option (which was lifted from the ssh2 library's own documentation).