Change definition of external rules

[nichtich]

• Allow an arbitrary JSON Object to specify external rules?
• Change name from checks to rules?
• Require rules to always be an array?
• Disallow rule names used in Avram specification

[nichtich]

• A external rule should either be an object or a string as shortcut for { "id": "...." }
• An external rule identifier should not match /^([a-z]+)([A-Z][a-z]+)+$/ (any camelCase string)

[nichtich]

Existing examples of external rules:

• shacl4bib rules in qa-catalogue
• pica-lint rules

[nichtich]

Done in 0.9.4