Introduce mechanism to parametrize standards.

[mbuechse]

Sometimes a standard can be neatly divided into a static part on the one hand and dynamic parts on the other, such as:

• scs-0104-v1, where the actual list of standard images is a yaml file that is not part of the standard text,
• the upcoming standard on connection security might refer to Mozilla recommendations, which get published as versioned json files.

The check scripts that are specified in the certificate-scope yaml may of course accept command-line parameters of all sorts, but those are not part of the official docs. If we introduce the concept of parameters for standards, their values could easily be documented.