Feature Request: Hierarchical File System and its export

Hackmd is an amazing project.
However, I think it will be better with the support of a hierarchical file system.

## Decription of Feature Request
It seems that the current notes are saved sequentially and displayed accordingly to users.
For example:
- hackmd-user1
- foo.md
- bar.md

It would be great to see hierarchy support like:
- hackmd-user1
- lecture_notes
- linear_algebra.md
- life
- must-go_resturants

## Potential Solution
It is plausible to handle this on one's own by providing "parent" in each header yml.
But I think it would be great to have this feature built in HackMD.