Structured metadata file for each episode

To keep this repo as the (kind of) single source of truth, and to aid in API building, I think it would be a good idea to keep some structured metadata file per episode. I used the YAML format for its human-and-machine-friendliness.

The publication date here allows, for example, the episodes.json to exclude episodes that have no publication date or a publication date in the future.

Closes #44 (closed)