Allow comments in lists

[arv]

I had the following md:

1. abc
  <!-- comment -->
  1. def

And I get the error message:

Possibly unhandled SyntaxError. Expected "  " or [0-9] but "<" found.

[bterlson]

I could probably parse this out on my end before passing to emd but emd also does not like blank lines:

1. a

2. b

will fail. Probably by design, too. So I could be smarter and remove whitespace around comments too but that seems prone to failure and more difficult than simply handling html comments in the emd grammar (which, afaict, shouldn't be too difficult).

[domenic]

Yep, agreed this should be handled by EMD.

We could also allow dedicated comment syntax, e.g. //.

[bterlson]

I think we need to support html comments at a minimum. I'm not opposed to a comment shorthand but I'm not sure it's necessary either :)