FR: update cff file instead of replacing it

(related: #15 (closed), !47 (merged))

Currently, only a new cff file can be created. However, since we now have the new-version feature for zenodo, it could be a useful extension to consider an existing cff file. Specifically, this refers to the identifiers section. See also the example in the cff docs over here. If a new cff file is created by facile-rs, it only has one identifier, e.g.

identifiers:
- description: This is the archived snapshot of version v0.1.11 of demo-facilers-to-zenodo
  type: doi
  value: 10.5072/zenodo.262888

However, it seems useful to me for versioned publications to keep track of the versions in the identifiers section, for instance like

identifiers:
- description: This is the archived snapshot of version v0.1.11 of demo-facilers-to-zenodo
  type: doi
  value: 10.5072/zenodo.262888
- description: This is the archived snapshot of version v0.1.10 of demo-facilers-to-zenodo
  type: doi
  value: 10.5072/zenodo.262871
...
- description: This is the archived snapshot of all versions of demo-facilers-to-zenodo
  type: doi
  value: 10.5072/zenodo.222521