Templates definition should include contactEmail key

[kerolasa]

The definitions in https://github.com/Domain-Connect/spec/blob/master/Domain%20Connect%20Spec%20Draft.adoc#52-template-definition should include contactEmail that is a string and expected to be an email address. Ideally the email should be a maillist or group that will reach people who can comment about the template, or be requested to make updates if/when there is an issue with a template. An issue could be minor syntax error such as TTL written as a string, or unreachable logo url.

The contactEmail needs to be an optional field to avoid backwards compatibility issues.