feature: imenu index

[VladimirAlexiev]

imenu is an interactive "table of contents" of a document. It's very useful for large docs.
Here's how it looks for a markdown doc (the window on the right):

To grok those dots, for comparison here's a generated TOC for the same file:

Looking at https://jblevins.org/projects/markdown-mode/ , you just need to define 1-2 functions:

(defun markdown-imenu-create-nested-index ()
(defun markdown-imenu-create-flat-index ()

And then in markdown-mode:

  (setq imenu-create-index-function
        (if markdown-nested-imenu-heading-index
            #'markdown-imenu-create-nested-index
          #'markdown-imenu-create-flat-index))