Add option to $sanitizer->truncate() to allow unclosed tags to be closed rather than removed

[Toutouwai]

Short description of the enhancement

$sanitizer->truncate() uses WireTextTools::fixUnclosedTags() to deal with any unclosed tags that might be present in the truncated string. But it doesn't provide a way to set the "remove" option for fixUnclosedTags() to false:

* When the remove option is false, it will attempt to close unclosed tags rather than 
* remove them. It doesn't know exactly where they should be closed, so it appends the 
* close tags to the end of the string. 

It would be useful to have an option for $sanitizer->truncate() so that unclosed tags are closed rather than removed.

Related forum topic: https://processwire.com/talk/topic/20602-truncate-removing-html-tags/