Add option for converting to (and ideally from...) .all-contributorsrc

[griff-rees]

I think there's a lot of overlap between this project and allcontributors, and wondering if some collaboration on that format would be beneficial. I'm in the process now of automating that for a research project to then release on zenodo. Relevant tickets:

• Issue integrating with all-contributors: Feature request/3rd party tool need: allcontributors-to-zenodo all-contributors/all-contributors#418
• Issue integrating with cff: add https://allcontributors.org/ cff-initializer-javascript#505
• Issue within our project lwmdb: Add all contributors bot Living-with-machines/lwmdb#125

FYI: @AoifeHughes