options: add --asc flag to sort in ascending order

Signed-off-by: Emilio G. Cota <cota@braap.org>
Add --nodowncase flag to avoid downcasing keywords

Signed-off-by: Emilio G. Cota <cota@braap.org>
Output the URL in the bib file, if available

Signed-off-by: Emilio G. Cota <cota@braap.org>
Add --keywords flag to only output for specific, ordered keywords

Signed-off-by: Emilio G. Cota <cota@braap.org>
Add author_date field to the output

This field can be used as a reasonable unique paper ID.

Signed-off-by: Emilio G. Cota <cota@braap.org>