Add folding support

[idvorkin]

Super cool project.

Folding would be a great feature for indented todos.

I think the folding on WOFL might be a similar capability.

https://github.com/lukaszkorecki/workflowish

[aserebryakov]

Well, this might be an interesting one to do since Vim actually supports folding itself.

[aserebryakov]

Implementation of this feature isn't as trivial as I would like to think.