Upgrade Markdown's ShouldSkipPaths as a global Config.ScanSkipPaths opti...

...on 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).

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This (partially) fixes the problem reported on uservoice.com (sorry for misusing that platform): http://servicestack.uservoice.com/forums/176786-feature-requests/suggestions/6502203-possible-performance-problem-in-markdownformat-plu

I noticed that the problem was already fixed in master, so I just applied the same changes in branch v3.

The fix seems not completely correct IMO:
When I set tell to skip a path, e.g:
  EndpointHostConfig.Instance.ScanSkipPaths.Add("/data/");
then no subfolders of /data/ will be scanned, but files inside /data/ will still be scanned and processed.

I don't know if this is by design or not. In any case, this commit fixes the performance problems reported on uservoice.com, since it stops scanning folders below the paths listed in ScanSkipPath.