Parameterize authorized_key options

This commit parameterizes many of the available key options for an
authorized_key entry. I have included the following options:

  - command
  - environment
  - from
  - no_agent_forwarding
  - no_port_forwarding
  - no_X11_forwarding

The existing parameter for key_options will take precedence over these
individual options, overriding them if both are provided.