use Omd_utils.html_void_elements

modified:   omd_backend.ml
modified:   omd_parser.ml
reverberate minor change in omd_backend.ml

description: now attribute values use `'` by default instead of `"`

modified:   ../tests/cow/html_tags.md
fix: partial fix in the choice of `'` versus `"' for attribute values

modified:   omd_backend.ml