Add multi-line comments for Elm (#283)

Technically the minimum necessary syntax for block comments requires
only a single hyphen (`-}`, `{-`), but I have found that using a double
hyphen plays a bit more nicely with the elmcast/elm-vim auto-formatting.
Further, uncommenting when using the single dashes leaves an extra level
of indentation (again, when using elmcast/elm-vim).

plugin/NERD_commenter.vim

@@ -159,7 +159,7 @@ let s:delimiterMap = {
     \ 'eiffel': { 'left': '--' },
     \ 'elf': { 'left': "'" },
     \ 'elixir': { 'left': '#' },
-    \ 'elm': { 'left': '--' },
+    \ 'elm': { 'left': '--', 'leftAlt': '{--', 'rightAlt': '--}' },
     \ 'elmfilt': { 'left': '#' },
     \ 'ember-script': { 'left': '#' },
     \ 'emblem': { 'left': '/' },