[core.Abstract] select_field: accept strings + fold kwarg

The fold keyword arg returns a single value if possible and a list
otherwise. Without that keyword you always get a list.
gca-filter: new command line tool to filter abstracts

Currently only supports direct comparisons
gca-tex: remove filter and sort function; use tools instead
[gca.tex] Add support for figures
contrib/convert.sh: small script for converting gifs to pngs