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 (#5).
• Un-wrapping erroneously wrapped lines (#7).
• Removing other <br/> tags that the current cleanup didn't remove (#9).
• Un-breaking the incorrect encoding of XML/HTML delimiters inside a <pre><code> block (#10).
• Tag duplications and erroneous <br /> tags when working with lists (#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).