Feature request: Get DOI from `DESCRIPTION`

[roaldarbol]

From the Writing R Extensions manual, it specifies that custom fields are allowed. That should mean we can add a DOI field to the DESCRIPTION file. (I might be wrong, but that is how I understand the manual - see the last paragraph in the linked section)

With that in mind, it would be great if cffr could check for DOI in the description file, as that would allow one to auto-update both CITATION.cff and inst/CITATION in the same step whilst only manually updating DESCRIPTION.