single-char markers in markdown-imenu

[VladimirAlexiev]

markdown-imenu-create-nested-index
         (empty-heading "-")
         (self-heading ".")

These single-chars are hard to hit with the mouse, so I'd change it to eg

markdown-imenu-create-nested-index
         (empty-heading "---")
         (self-heading "...")

[syohex]

I'm not sure that how about introduce customize variables like this https://github.com/jrblevin/markdown-mode/commit/4854179b789d67ef6134db5ae2cb9126ae0be993 ?