Restrict access to only authorized_users

This adds a new configuration option `authorized_users` that should contain an array of emails. After authentication via OmniAuth, the user's email is compared against this list and an exception is raised if their email is not included. Leaving `authorized_users` blank or setting it to an empty array (the default) disables this functionality.

I liked that Omnigollum added authentication to Gollum, but I was disappointed to find that there apparently was no option to only allow certain users to have access. This change adds that.