Use regex in path to match subdirectory?

Looks like the loadPath only accepts string or lookup function and the [interpolator](https://github.com/i18next/i18next-node-fs-backend/blob/master/src/index.js#L84) only populates language and namespace option. There seems not to be a way to match files in subdirectory via regex. I'm wondering how to elegantly handle look up in subdirectories since I would like to organize my translation files into subdirectories rather than putting them all into one big directory.

Read src/index.js around the interpolator at line 84 and inspect how loadPath accepts strings or lookup functions. Determine a supported way to locate translation files in subdirectories, with completion defined by subdirectory-organized translation files being discoverable through the backend.

Written by the indexing model from the issue text.