Add new action for auto-reply

[ranmocy]

When the condition is matched, we could automatically sent an email to the sender with pre-defined message.

[ranmocy]

Related to forwarding an email #33.
I think we could provide an generic "send_email" action.

Idea of syntax could be a list of actions:

(forward-to "someone@gmail.com") / (reply-to (all/default/sender))
(title "override title")
(body "my reply" (quote (original_body)) "signature")
(cc "additional@gmail.com")

This may vary depends on what APIs are available.

Supporting format in body is tricky, it's preferred to use (if Gmail supports) templates instead.

[SullieLore]

Would be very handy to have this implemented. Excellent project by the way.