Support ids field in bibtex files

[Omikhleia]

Bib(La)Tex has an interesting ids field:

  ids = {alias1, alias2}

"Citation key aliases for the main citation key. An entry may be cited by any of its aliases ... the citation (will be treated) as if it had used the primary citation key. This is to aid users who change their citation keys but have legacy documents which use older keys for the same entry."

[Omikhleia]

The whole rationale is indeed

"This is to aid users who change their citation keys but have legacy documents which use older keys for the same entry."

As I am extending my bib files from various sources, I realize there are some inconsistencies in key names. So far so good (I haven't released anything final), but at some point, it will become a compatibility issue.