Hello!

As I was modifying gnus-score.el, it occurred to me that a way to
specify user-defined scoring functions could be useful in cases where
even advanced scoring isn't sufficient. I put together some code and
documentation for that.

Although it's only ~40 lines of Elisp and ~30 lines of Texinfo, I am
pretty sure it's the largest code change I have submitted to Emacs and I
would not be surprised if I violated some coding standards. I have spent
a fair amount of time with testing, but cannot rule out corner cases, of
course. Let me know if you want me to make any improvements before
accepting this patch.

Thanks!