Feature Request: Folder organization

It seems like lots of people still need the **folder feature**. Especially when you have tons of notes and run many teams/projects. 😢

However, one of the related issue has been closed #442, because @SISheogorath pointed out that the change are too invasive. Indeed, the implementation will effect all the notes CRUD behaviors, so I'm wondering if there is an easier way to do it. 🤔

**For example**, use the markdown like `###### tags: ` but replace to `dir`:

`###### dir: {folderName}` or
`###### dir: {folderParent} > {folderChild}` or
`###### dir: {folder_1} > {folder_2} > {folder_3}`

Next, let the front-end to do magic tricks! Maybe we don't need the **real entity** to seperate folders. Just use UI to virtualize it. Also, that will be able to split the permission concept which does not need implement on the folder right away.

**Screenshot** (modified from ShowDoc UI):


CodiMD rocks ! 💯
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The following issues might be related:
> #12
> #350
> #442
> #949