fix: allow HTML blocks to directly follow HTML blocks

Now an HTML block can either be:
- **(unchanged)** the very first element (nothing precedes it)
- **(unchanged)** the first element on a Markdown line
  (note that in `> A\n> B`,
   B is the first element of the second line
   of the blockquote)
- **(new)** an element that directly follows another HTML block
  (in this case, no character may separate them,
   not even a space)

This should fix https://github.com/ocaml/omd/issues/112