Add a `$run_texturize` static var to `wptexturize()` that is filterable via a new run-once filter: `'run_wptexturize'`. Allows user to disable texturization.

Needs filter docs.

Props nacin, SergeyBiryukov.
See #19550.


git-svn-id: https://develop.svn.wordpress.org/trunk@28715 602fd350-edb4-49c9-b593-d223f7449a82