Sublime Text is one of the best plaintext editors available, very flexible and extendable with hundreds of plugins. Unfortunatelly most of them are for programmers, but writers, researchers or ordinary users can also find something for themselves.

You may be interested in this fresh Outline Plugin. Although it's very limited in its functionalities, you may combine it with other Packages (Filebrowser, MarkdownEditing, LinkOpener). It is fast, reliable and free to try

https://packagecontrol.io/packages/Outline