add support for M4 macro processor (#271)

M4 knows about two types of ignoring text.  But the concept which the
GNU M4 manual refers to as "comments" [0] (i.e., lines prefixed with
hash signs) is actually telling the interpreter "ignore this text, but
pass it to the output".  Most of the time, people don't want the macro
comments in their output, so instead the `dnl` ("discard to next line")
macro [1] is usually used for comments in the document-my-thoughts
sense.

[0]: https://www.gnu.org/software/m4/manual/m4.html#Comments
[1]: https://www.gnu.org/software/m4/manual/m4.html#Dnl

plugin/NERD_commenter.vim

@@ -255,6 +255,7 @@ let s:delimiterMap = {
     \ 'lua': { 'left': '--', 'leftAlt': '--[[', 'rightAlt': ']]' },
     \ 'lynx': { 'left': '#' },
     \ 'lytex': { 'left': '%' },
+    \ 'm4': { 'left': 'dnl ' },
     \ 'mail': { 'left': '> ' },
     \ 'mako': { 'left': '##' },
     \ 'man': { 'left': '."' },