Use HTML comment syntax for Rmarkdown

Closes #415

While Markdown doesn't officially have a comment syntax, HTML comments
are widely parsed as such. This is what most Markdown treat as Markdown
comments. As a Markdown derivative it seams logical to do the same thing
in Rmarkdown.

The previously added '#' syntax seems to be for inside code blocks, and
that would be language dependent anyway. I've left the hash option as
the alternate.

plugin/NERD_commenter.vim

@@ -370,7 +370,7 @@ let s:delimiterMap = {
     \ 'resolv': { 'left': '#' },
     \ 'rgb': { 'left': '!' },
     \ 'rib': { 'left': '#' },
-    \ 'rmd': { 'left': '#' },
+    \ 'rmd': { 'left': '<!--', 'right': '-->', 'leftalt': '#' },
     \ 'robot': { 'left': '#' },
     \ 'robots': { 'left': '#' },
     \ 'rspec': { 'left': '#' },