Providers are modules that export:
    CONFIG = {
        "config key": "optional | required",
        ...
    }

    update(config, state) -> list[items]

And optionally:
    action_NAME(config, state, item) -> None

    on_create(config, state, item) -> None

    on_delete(config, state, item) -> None

Sources are folders in the data directory with:
    config: user-modified configuration values, drawn from the provider
    state: provider-modified data
    *.item: feed items

config reserved keys:
    "provider": the provider the source uses
    "name": given to sources by default, individuates sources from same provider

GET /config
    Edit subfeed configs

GET /config/[feedname]
    Edit config values for feedname

GET /feed/[feedname]
    Feed view for feedname

<!-- GET /items
    Get item JSON
    ?include=[tags]
    ?exclude=[tags]
    ?limit=100 -->

CLI:
    intake create [provider] [--config KEY VALUE] [-c KEY VALUE] ...
        load the provider
        verify that the provider's config is satisfied
        create the source and set the config values

    intake update [source] [--reset]
        load the source's config and state
        use config.provider to load the provider
        verify the source config against the provider config
        call provider.update(config, state) to get current items
        merge new items into current items
            if --reset, new items overwrite everything, including active

    intake deactivate [source] [--tag TAG] [--title TITLE]
        load each item in the source and deactivate it

    intake add [--id ID] [--title TITLE] [--link LINK] [--time TIME] [--author AUTHOR] [--body BODY] [--tags TAGS] [--ttl TTL] [--ttd TTD] [--tts TTS]
        create the item and add it to the default source

    intake feed [--json]
        Dump active item feed to stdout

    intake action [source] [item] [action]
        load config, state, item
        verify item supports this action
        load config.provider
        verify config
        verify provider supports this action
        action_[action](config, state, item)