Add support for MX validation with an A record fallback.

Hi,

We're using your email validation gem to serve 6000+ bookings a day on our site. Unfortunately we've had a couple of instances where customers have rung up complaining we're not accepting their valid email addresses aren't able to register because we're validating the mx record of their domains using your gem. 

However, these customers are emailable because most SMTP servers will (and should according to https://en.wikipedia.org/wiki/MX_record#History_of_fallback_to_A) fall back to the DNS Address (A) record.

This PR adds support for a `:mx_with_fallback` that validates either MX **or** A record.