Support arbitrary file names

[alurm]

Currently, only UTF-8 file names are supported (since that's what JSON expects). In some cases, file names are not UTF-8. This use case might be worth supporting at some point in the future.

[alurm]

A possible way to implement this is to add a directory directive.

- directory
-
  # Entry's name, base64'ed.
  - [base64, ...]
  # Entry's value.
  - ...