check-ssl-host plugin

Connects to a HTTPS (or other SSL) server and performs several checks on the certificate:

- Is the hostname valid for the host we're requesting
- If any certificate chain is presented, is it valid (i.e. is each certificate signed by the next)
- Is the certificate about to expire

There are a couple of other plugins that work with SSL certificate, but other plugins focus mostly on expiry, and don't support SNI (except for the net/http based ones in ruby 2.0 and above).