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

syntax/outline.vim

@@ -0,0 +1,29 @@
+" =============================================================
+" Description:  Syntax file for GNU Emacs' Outline mode
+" Author:       Alex Vear (axvr) <av@axvr.io>
+" Licence:      ISC (2019)
+" =============================================================
+
+if exists("b:current_syntax")
+    finish
+endif
+
+" Enable spell check for non syntax highlighted text
+syntax spell toplevel
+
+" Headings
+syntax match outlineHeading1 /^\s*\*\{1}\s\+.*$/ keepend contains=@Spell
+syntax match outlineHeading2 /^\s*\*\{2}\s\+.*$/ keepend contains=@Spell
+syntax match outlineHeading3 /^\s*\*\{3}\s\+.*$/ keepend contains=@Spell
+syntax match outlineHeading4 /^\s*\*\{4}\s\+.*$/ keepend contains=@Spell
+syntax match outlineHeading5 /^\s*\*\{5}\s\+.*$/ keepend contains=@Spell
+syntax match outlineHeading6 /^\s*\*\{6,}\s\+.*$/ keepend contains=@Spell
+
+hi def link outlineHeading1 Title
+hi def link outlineHeading2 outlineHeading1
+hi def link outlineHeading3 outlineHeading2
+hi def link outlineHeading4 outlineHeading3
+hi def link outlineHeading5 outlineHeading4
+hi def link outlineHeading6 outlineHeading5
+
+let b:current_syntax = 'org'