[feature] support external classifiers for tagging

[masukomi]

As a user I want to be able to used automated classifiers to tag taggable things with appropriate tags I may have forgotten or not bothered to add so that I can be assured that a taggable item will show up under expected tags when I go looking for it.

Implementation Thoughts

YAML configuration file for external classifiers
Classifier must be able to take an url, or a path or both. It must print a valid JSON array of strings to Standard Out which BB would process and add as tags to the object.

URL would be the url of the thing (assuming a bookmark, or future taggable things with associated urls) PATH would be to the MD file of the archived item.

Classifiers would be invoked during an ExternalClassificationJob that would run after a bookmark has been archived.

I think if ArchiveUrlJob fails then it could check for classifiers that don't need PATH and kick it off for those.

I think at startup we initialize a bunch of ExternalClassifier objects in memory from the YAML. Maybe accessed via a Classifiers singleton? Which could have an .url_only and .path_only and .url_and_path methods which returned you an array (possibly empty) of classifiers that met the required criteria.

Maybe - for future proofing - any of them can also take the ID and OBJECT_TYPE (snake_case value like social_media_account) and then they can pass stuff back to the system via the API that doesn't exist yet. They'd need an API key which also doesn't exist yet.

foo-classifier:
  name: "foo"
  needs:
    - URL
    - PATH
  executable
    path: /path/to/foo
    arguments:
      - "--url"
      - "{URL}"
      - "{PATH}"