Feature: Valid SSL rule

[phanan]

To follow up #12, as suggested by @barryvdh: Implement rule to validate SSL. It's 2017, so every site should come with a valid SSL certificate. We might as well issue a notice/warning if the cert is expiring in less than 30 days.

This package looks promising, but requires PHP7+. As such, if this rule is implemented, kupo must be bumped up one major version.