add plugin API for cleanup functionality

[garretwilson]

There are so many cleanup-related things that need to be done to the BlueGriffon generated source code:

• Fixing line breaks (inappropriate addition of LF line wrapping on Windows machines #5).
• Un-wrapping erroneously wrapped lines (line wrapping setting ignored #7).
• Removing other <br/> tags that the current cleanup didn't remove (cleanup doesn't remove trailing <br /> tags #9).
• Un-breaking the incorrect encoding of XML/HTML delimiters inside a <pre><code> block (blocker encoding bug breaks XML compliance #10).
• Tag duplications and erroneous <br /> tags when working with lists (copying <dfn> into a list improperly duplicates tags #12).

In light of all the things needing cleaning up (several of them blocker issues), and recognizing that you already have a "cleanup" dialog to clean up other items, perhaps you could add an API to the clean up facility to allow us to plug in our own cleanup code (since it appears that actually fixing these bugs is a long ways off).