Authenticate the email endpoint if auth_token is configured.

If an auth token is configured the webhook must have the token present in the params as `:token` to stand a chance of being successful. 

I think this may be good if someone who is malicious guesses an app is using Griddler with the default endpoint. 

Part of me thinks this may not be necessary if your email processor is checking the `to` param for certain tokens. However, someone may be less strict and just check the `from` to take action. 

What do you think?