[FEATURE] Keep tags when stripping empty tags in HtmlParser

A new option for the HTMLparser.stripEmptyTags configuration
is added:

HTMLparser.stripEmptyTags.keepTags = tr,td

This will make the HTMLparser remove all empty tags except the
configured ones. If this setting is used the .tags configuration
will have no effect any more.

Resolves: #72045
Releases: master
Change-Id: I7137db597e442460aa7add9d99bdf73db0d5cbd5
Reviewed-on: https://review.typo3.org/45107


Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>