Uses validatorjs to validate rules defined in apikana-defaults (under 'validation').
See the validatorjs documentation on how to define such rules.
The main intention behind this feature was to enforce the presence of
additional properties in an OpenAPI specification.

Appends a paths metadata to each of its methods description. Relies on an object in apikana-defaults, "metadata", containing the properties "matchProps" and "mappedKeys".
"matchProps" expects a string containing a regular expression matching the paths properties which should be handled as metadata, the default being "^x-.*$".
"mappedKeys" expects an object containing mappings for the keys contained in a metadata property, intended to make the descriptions better looking, the default is an empty object.