PSR-0'd the new 'txp:if_yield' tag.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5343 2fea6a4d-a838-0410-917b-93a53c48e9d2
'txp:if_yield' compares values strictly as strings.

Avoids type juggling issues. This allows telling the
differences between numeric-like values or
'empty' bytes.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5344 2fea6a4d-a838-0410-917b-93a53c48e9d2