Add port of GNU Emacs' Outline mode

<https://www.gnu.org/software/emacs/manual/html_node/emacs/Outline-Mode.html>

ftplugin/outline.vim

@@ -0,0 +1,16 @@
+" =============================================================
+" Description:  Folding for GNU Emacs' Outline mode
+" Author:       Alex Vear (axvr) <av@axvr.io>
+" Licence:      ISC (2019)
+" =============================================================
+
+function! OutlineFold()
+    let l:depth = match(getline(v:lnum), '\(^\*\+\)\@<=\( .*$\)\@=')
+    if l:depth > 0 && synIDattr(synID(v:lnum, 1, 1), 'name') =~# 'outlineHeading'
+        return ">" . l:depth
+    endif
+    return "="
+endfunction
+
+setlocal foldexpr=OutlineFold()
+setlocal foldmethod=expr