Options for custom authorship fields

[laurelfulford]

This came up in the discussion in #8 from @jeffersonrabb:

In past experience with editorial organizations, custom authorship fields are a common, and complex need. Examples include joint authorship, art contributors (photos by, illustrations by), co-productions with other publications, and more. Newspack will need to settle on a single solution for this, and we’ll need to figure out how to support these fields in homepage blocks.

To start, we need some exploration into what options are available out there. The resulting recommendation will then need to be integrated into the Homepage Articles and other Newspack blocks, as well as the theme.