Support for feed folders with any level nested

[tseykovets]

It seems that YARR does not currently support nested feed folders. This becomes a problem when working with a very large number of feeds, as well as when importing feeds from other RSS readers, where they were structured into folders with several level nested.

It would be nice to implement support for feed folders with any level nested. For example:

GitHub
	Go
		Branches
			master
			dev.coretypes
			dev.inline
		Releases
		Tags
	YARR
		Branches
			master
			medialinks
		Releases
		Tags

Attached is the feeds.txt (need to change extension to opml) file with this feeds structure for testing and experimentation. Currently, importing such a folder structure is lossy.

For convenient work with folders with any level nested, it is better to implement the feed list as a tree view. In this case, please pay attention to accessibility. See https://www.w3.org/WAI/ARIA/apg/patterns/treeview/