--split-exp should be able to generate subdirs

**Please describe your feature request.**

Splitting documents into mutliple files is neat. Being able to specify a subfolder that can be created on the fly would make this even neater. Use case is where splitting miht result in a lot of of files, and akes it easier on the human.

**Describe the solution you'd like**

Could via an additional flag to allow subfolders to be created.
