(instead of just printing them, might be useful for scripts — also, I've
not come up with a better name than "nixq" yet, so it really ought to be
able to query things)

Not sure yet if i'll keep the printing code, it's kinda fragile &
there's almost definitely a crate which i could use instead.

this incidentally also moves a lot of the parsing logic out of
piplines.rs and instead keeps it in queries.rs, where it should probably
be anyways, so the pipeline module can focus on just … well, applying
the filter pipeline

since this is apparently enough to edit all of nixpkgs to remove
lib.mdDoc without (?) breaking anything, I think it should qualify as a
tiny, tiny release.

just a skeleton to see if this works

with thanks to pennae, who showed that this approach was possible
(and wrote the status-report widget)
 https://github.com/pennae/nix-doc-munge