Add option to overwrite specific fields

[aarontaycheehsien]

I understand bibfix currently never overwrites any values.

There might be use-cases where you want to specify some fields that can be overwritten.

For example, say you extract records from Google Scholar, using Harzing's Publish or Perish in RIS.
While the RIS abstract field might seem to have 100% completeness, this isn't really the "abstract" but the GS display snipplet.

Would be good if Bibfix gave an option to overwrite totally specific fields.