Add SSL Test

[jbmoelker]

ssllabs.com/ssltest/ grades the SSL of each server in the response chain of a page request. It grades certificate, protocol support, key exchange and cipher strength.

There's an API and node modules available.

Possible issue is that a scan on SSL Test can take quite some time slowing down the entire Lighthouse run.

Note: we'll need some way to convert letter grades to numerical values so we can use them for audit score calculations.