bug#3425: 23.0.94; Please make flyspell-generic-check-word-predicate a hook

I suggest that flyspell-generic-check-word-predicate is made a hook.
It is quite inconventient to add new checks as it currently most be a
function. There is for example a question on EmacsWiki for how to
avoid checking words in an URL. That would be a good candidate to add
to a hook.

Converting it is easy. It is just replacing the lines below

 	      (and (fboundp flyspell-generic-check-word-predicate)
 		   (not (funcall flyspell-generic-check-word-predicate))))
with
              (run-hook-with-args-until-success
flyspell-generic-check-word-predicate))

As hooks are defined now old code will continue to work.


In GNU Emacs 23.0.94.1 (i386-mingw-nt5.1.2600)
 of 2009-05-23
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4) --no-opt --cflags
-Ic:/g/include -fno-crossjumping'