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: https://github.com/all-contributors/all-contributors/issues/418
• Issue integrating with cff: https://github.com/citation-file-format/cff-initializer-javascript/issues/505
• Issue within our project lwmdb: https://github.com/Living-with-machines/lwmdb/issues/125

FYI: @AoifeHughes