Syntax highlight and correctly format/manipulate lists

Adds support for syntax highlighting and formatting both ordered and
unordered lists (excluding unordered lists delimited with `*`, as they
conflict with headings and are generally not recommended).

e.g.

    1. Foo
    2. Bar
       1) Biz
       2) Baz

    - Foo
      - Bar
    + Biz
      + Baz

ftplugin/org.vim

@@ -11,6 +11,9 @@
 "   Web: <https://orgmode.org/manual/index.html>
 
 setlocal commentstring=#%s
+setlocal comments=fb:*,fb:-,fb:+,b:#,b:\:
+setlocal formatoptions+=ncqlt
+let &l:formatlistpat = '^\s*\(\d\+[.)]\|[+-]\)\s\+'
 
 setlocal foldexpr=org#fold_expr()
 setlocal foldmethod=expr