pass extra context to invitation email by using a custom Devise::Mailer ...

...and changes to devise_invitable/model.rb

Pass additional context to :invitation_instructions that use *args from recent send_devise_notifications. These changes are needed to work with devise-async to pass down extra parameters when they can't be persisted (attr_accessor on user).

Example implementation:
https://gist.github.com/bruchu/6889955