AWS plugin: add MFA device handling

Added a couple of commands for MFA device handling using oathtool.
- aws_mfa_add_device: adds a virtual device, encrypting the secret seed
  using gpg and storing it in $AWS_HOME/account_name.mfa.asc.
  It also auto-completes profile names, so the account is associated to an
  existing profile.
- mfa: generates an MFA code based on the encrypted secret seed. It requires
  oathtool to be installed, and optionally integrates with the clipboard
  using pbcopy (on Mac OS X) and xsel (on X11 systems)

This is also based on code shared to me by @elias5000 and slightly refactored to by yours truly be more generic.