Allow completion for predicate filters

Ripped from `eval-expression`, I'm not sure about the fourth `t` argument for `read-from-minibuffer`, but it seems to work... can you confirm before pulling?

Also, do you have an idea to show the list of available keywords (isdir, name, uid, etc)? In dired+ they make it part of the prompt but imho it'd become a giant prompt. Maybe a custom keymap with `C-h something`?