wai-app-static: Allow accessing .well-known

[dpwiz]

Currently the static app can't be used to serve Let'sEncrypt challenges as it refuses to serve hidden files.
While it is possible to provide custom ssLookupFile, making such a function is quite inconvenient as it implies building the File object.
A simple flag to allow the directory or a list of allowed prefixes would be much nicer.

[Vlix]

Could you link to the code section that filters out hidden files?

[dpwiz]

@Vlix

wai/wai-app-static/WaiAppStatic/Storage/Filesystem.hs

Line 196 in b6bc2b6

entries' <- fmap (filter isVisible) $ getDirectoryContents fp