resolves #347 allow custom converters configured in playbook
This adds a "converters" section to the asciidoc section in the playbook, similar to the extensions section. Entries are modules to load as converters or "transforms". Such a module must export a function
(baseConverter, context) => {object containing transforming functions}
The context
object contains {file, contentCatalog, asciidocConfig}.
The converters are chained together, ending with the base converter (the existing Antora html5 converter).
Possibly the section in the playbook should better be called transforms
?