Propose modX->filterPathSegment() method

My proposed solution to http://tracker.modx.com/issues/10222

This is an alternative to the approach proposed by modxcms/revolution#620 that would allow modResource derivatives to extend the behavior.

I also wanted to change the semantics, since we are filtering or transforming more than cleansing IMO. Also, alias is not descriptive enough IMO, so I would like to move toward calling it what it is, a path segment as defined by URI/URL specifications.