App extension: Close all open docs

[ChrisClems]

Add an application extension method that will check for open documents and close all.

This is useful for background applications that must iterate over large numbers of parts while handling exceptions gracefully across many code paths. Leaking unclosed documents will very quickly degrade performance. When execution must continue in the face of any handleable exceptions, purging all open documents will ensure you don't come back to your script moving at a crawl due to dozens of open documents.