utf8_stripspecials optimization

Add preconverted utf-8 string of special characters.

The (once only) conversion of the special character  unicode
array into utf-8 occurs on every DokuWiki page view,
irrespective of action or caching, and takes about one third
of the time involved in delivering a wiki page straight from
cache.

The original unicode array has been left in place in the file
to make any future amendments easier.

darcs-hash:20060923151937-9b6ab-cae0340a95d9596415ef71d7b7e67ef9daca84ef.gz