[Feature Request] Ability to sort your subscriptions into folders and sub-folders

[hffaust]

• I am imagining a way to group subscriptions of a certain type and sub-type. I think this would be very useful.
• Say for example, I have subscriptions to different news outlets all over the world and I want to sort my subscriptions by continent and then country and then city... I could create a folder structure like the following and then be able to easily find articles while minimizing space in my side bar (I am envisioning this folder system to be collapsible, just like the current subscription listings).

World News
|--- Europe 
       |---France 
       |     |--- Paris
       |     |--- Marseille
       |
       |--- Germany
       |     |--- Frankfurt
       |     |--- Berlin
...

• Before using Raven Reader I was using Vienna RSS reader (Mac only), and they have implemented a pretty good subscription folder system.
  • Reference for ideas: https://github.com/ViennaRSS/vienna-rss