fix: (partial fix) performance issue with HTML

modified: omd_parser.ml
  * fix an optimisation that went wrong in the new HTML parser
  * todo: do something for wrongly formed HTML? Or give a specific treatment
    to `img` tags?
minor fix: HTML output: make img, br and hr self-closing tags

modified:   omd_backend.ml
Merge branch 'patch-1' of https://github.com/Chris00/omd into HEAD