Allow hidden files to be accessed when an environment variable is set

[moqmar]

I'd like to also serve ACME/Let's Encrypt challenges through hastatic, but as the directory has to be called /.well-known/acme-challenge and has to be reachable from the internet, that's currently not possible.

I'd suggest to add an environment variable ALLOW_HIDDEN which if set to a non-empty value that's not "0", "no", "n" or "false" (case-independently) disables the protection of hidden files and directories.