allow for adding ssh extensions to certificate

[mnestor]

With the support to getExtensions to show the extensions of ssh certificates there should also be support for creating those extensions as well.

Hopefully someone can help clean this up. Example of how to create the extenions

var cert = sshpk.createCertificate(ids, userKey, host, signingKey, {
    lifetime: 300, // 5 minutes
    extensions: [
      'permit-X11-forwarding',
      'permit-agent-forwarding',
      'permit-port-forwarding',
      'permit-pty',
      'permit-user-rc'
    ]
  })