fix list items parsing (#62)

the unindenting algorithm for list items was slightly incorrect,
this commit fixes it. Basically it was sometimes cutting too soon.
add several characters to be escaped when outputing markdown
fix omd_backend.ml+omd_parser.ml

omd_backend.ml:
- improve markdown output
- improve markdown paragraphs

omd_parser.ml:
- add proper escaping for `>` and `<`