Add working implementation of bibtex-ignore-keys

[alejandrogallo]

I needed a setting to not export the abstract of my big library, so that other programs
have a better time parsing it.

I think this is a useful addition to the bibtex settings, what do you people think?

[alexfikl]

Looks good to me!

I keep deleting the abstract from my bib files too, so this is definitely useful.

[jghauser]

This is neat and makes it much easier for me to get rid of dois, which I tend to have little use for in my bibliographies :)

[alejandrogallo]

great !