Support a `group by: Org` option

[damithc]

Current:

Suggestion: Also support an Org option.

Justification: This will help to group together contributions to the same project but across different repos within the same project (some projects have multiple repos for documentation, testing etc.)

Notes:

• This may be harder than it appears, if this affects internal processing flows and data structures
• If this is added, blurbs.md should be able to support per-org blurbs as well (but it can be a separate PR)