Re-commit r34072 with some modifications:

Re-commit r34072 with some modifications:
* turned off by default (set $wgAdvancedSearchHighlighting to turn on)
* reverted r26269, \b doesn't interact very good with unicode data,
  so it broke highlighting of words that end/begin in nonascii chars
  completely
* small bugfixes in unicode handling, tested in more languages
* $wgSearchHighlightBoundaries need to be set to "" for CJK wikis
* benchmarking: on typical simplewiki data, the code is around 4-5 slower
  (according to noc.wikimedia.org the old code profiles to about 0.8%),
  but can be up to 20 times slower on featured-size articles
* update release notes (also for r33400)
* fix profiling errors in SpecialSearch