Add SNI support

[mallardduck]

This will require more investigation though it will be a useful feature.

SNI support is initiated by the client as long as a server supports it. If we can add support for SNI that means someone can provide both a domain and an IP to test with. Using those we can test the IP for the provided domains SSL.

This will be useful in migration scenarios where the DNS may not be live yet.

[mallardduck]

Added info:

http://serverfault.com/questions/506177/how-can-i-detect-if-a-server-is-using-sni-for-https

https://sni.velox.ch/

http://stackoverflow.com/questions/12941703/use-curl-with-sni-server-name-indication

https://curl.haxx.se/mail/archive-2015-01/0042.html