Regex for directory validation

[tyarkoni]

At present, the validator doesn't validate directories, only files. I gather from @chrisfilo this is because directories can't be uploaded in Chrome. But since the regexes stored here are going to be language-agnostic, it would be very helpful to have a regex that validates directories. Indexing with pybids can get pretty slow, because, e.g., the .git directory gets scanned for valid files even though it could be ignored.