use schema for feed_config

[gildesmarais]

• [ ] make use of dry-schema for feed config (channel + selector etc)
• [ ] validate against schema
• [ ] have each extractor/postprocessor define their schema
• [ ] export json scheme with https://github.com/ianks/dry-schema-extensions/blob/master/docsite/source/extensions/json_schema.html.md
• [ ] h2r-c: add modeline # yaml-language-server: $schema=https://my.url.to/the/schema to existing configs
• [ ] h2r-c: and to generator-generated configs