add Omd_utils.html_void_elements

thanks to @dsheets
https://github.com/ocaml/omd/issues/127#issuecomment-58437864

modified:   omd_utils.ml
modified:   omd_utils.mli
use Omd_utils.html_void_elements

modified:   omd_backend.ml
modified:   omd_parser.ml