Add support for duration time strings on session middleware config

This basically allows adding duration settings as strings, which
are then parsed by moment() to determine the duration. For example:

app.use('session', {
  regenInterval: "5 minutes",
  permanentExpires: "1 month",
  defaultExpires: "1 day",
  temporaryExpires: "24 hours",
  guestExpires: "1 week"
});