Tweaks to generated markdown

Hi Ulf,

Not sure if you will care for all or some of these patches. I've only tested them on an `overview.edoc` in one of my projects where I've started to experiment with making use of edown. That said, I think they improve the generated markdown.

* Remove unused imports from merl_lib in edown_xmerl
* Generated github-style "fenced" code blocks instead of `<pre>` tags.
* Fix "close" of markdown headers, octothorpe counts should match
* Adjust newline behavior for generated markdown. I see there is an existing issue/PR for this and am unclear if what I've done somehow breaks some edge cases. To my eye, the generated markdown is easier to read and I think will be "correct" for github use (I put that in quotes b/c markdown is markdown).