config: load all documents from a directory

[karenetheridge]

as suggested in karenetheridge/OpenAPI-Modern#51

A config value such as 'schema_dir' - iterate through all files in this directory (recursively), attempt to decode them (yaml or json), and create then as OpenAPI document objects to load into the evaluator object.

By default, assign them identifiers that are their relative path names within the directory.

[karenetheridge]

can also do import: [ url, url, url ] which will load from the network the supplied URLs and add them as documents. It is expected that the Content-Type should be application/json, text/yaml or (preferrably) application/openapi+json, application/schema+json or application/openapi+yaml etc.