Ability to automatically remove unknown macros/templates

Would be good if the user could decide to preserve or remove all unknown macros/templates.

As tmortagne suggested on the forum, preserve should create a blank macro definition that initially displays an "undefined macro message". This would then allow the macro to be created a posteriori or give the user the ability to set output to blank.

More info here: https://forum.xwiki.org/t/filter-streams-help-mediawiki-import/3586/3?u=ben.megson