Allow disabling of author names on listing pages

[apreshill]

For team sites it is a great feature to see content authors listed on the listing pages, but for individual sites it can feel self-congratulatory to list your name over and over again. With distill, we enabled this in the _site.yml file- author names are printed by default but you can disable this with:

---
collections:
  posts:
    authors_metadata: false
---

It would be great to be able to do this on a site- and per-post level, and I think relatively easy to do with Hugo.