There are other expansions available, but I don't really think they make
sense, or why there are two of them for the same thing (both $FILE and %
expand to the path).
I added $f and $p, which expand to the document's path and current
page respectively, while keeping the other 2 just in case it breaks
someone's setup. I also documented them in both zathura(1) and
zathurarc(5).