Optionally support Multi-Factor Authentication by allowing
(SET_)MFA_SECRET to be configured to contain the Amazon MFA secret.
If Amazon is configured to use Two-Step Verification, additionally supplying a one-time password in the authentication data, by-passes CAPTCHA verification.
Support has been added in a backwards-compatible way by allowing the MFA secret to be optionally configured. If the MFA secret is configured then 'oathtool' is used to generate the one-time password.
