Fix calls to `filter-buffer-substring'.

(Just remove the two extra arguments, since this looks like the original
intention.)
Lots of small style changes.

Since Elisp people tends to be extremely strict about things like line
lengths etc.
More improvements.

Mainly using backquotes instead of harder-to-read code.
Get rid of useless `listp's.

It really just checks cons-or-null.
Use `delete-dups' instead of `remove-duplicates'.

Still bad for the same reasons, so keep the comment.