* Allow users to filter directories by ending their path with "/"
* Allow users to filter with a Regexp, some scenariors can really require it.
* Use Pathutil#in_path? for Symlink verification, it real/expand.
This also requires some downstream work in "jekyll-watch" which at this time is
not very robust, it doesn't recognize the difference either, and should probably
start doing so (what I mean is detecting "/" and using the full path.)