Upgrade Markdown's ShouldSkipPaths as a global Config.ScanSkipPaths option which VFS now avoids scanning.

Upgrade Markdown's ShouldSkipPaths as a global Config.ScanSkipPaths
option which VFS now avoids scanning.
This change corresponds to commit
16f36d061472162b61825c6f69e1b8a486b372f1 on master (by mythz, Nov 26,
2013).