Implement keywords field from BibLaTeX

[lukeflo]

I started implementing the Hayagriva format for my little tui App for managing/interacting with bibliographic files/databases and encountered that there seems to be no equivalent of the keywords field from the BibLaTeX approach. At least, I couldn't find such a method as part of the Entry struct in the docs.

Such a field seems unnecessary for creating printable references through CSL styles, but it is very valuable for managing your bibliographic files; e.g. if you want to filter entries by topics etc. My tui, for example, makes heavy use of the keywords to filter your BibLaTeX database fast and uncomplicated.

Thus, it would be nice to see this method also implemented for Hayagriva!