Allow override of provider sign-in logic

[zorab47]

Override the default sign in behavior of Devise (and later other providers) by configuring a AnyLogin.sign_in proc to execute. This is a proof of concept for further discussion about issue #14.

Example

AnyLogin.setup do |config|
  config.sign_in = proc do |loginable|
    # Assume permissions of another user instead of logging in as that user
    current_admin_user.update_columns(assume_permissions_of_admin_user_id: loginable)
  end
end