v0.6.0

Breaking change: removes the `plaintext` artifact and the `beautifulsoup4` dependency. The stored plaintext was derived from the rendered HTML, so the same information is recoverable from `rendered_html`.

Removed: the `plaintext` field on the public `Snapshot`, the `snapshot.plaintext` database column, and the stored `.txt` blobs. Regenerate your migrations from `pravda.db.Base` to drop the column; previously stored `.txt` files are no longer referenced and can be pruned.