Add Pluggable Scanner Spec version 1.2 support

[pederhan]

The spec is located here, while the new README section describes how it works.

In general, it's a lot to take in, and we need to find out whether or not this new spec is something we have to support alongside the existing 1.1 spec, or if we can just migrate to 1.2. I am afraid it's the former. I will try to make time to figure this out ASAP.