omd_parser.ml: fix: typo in uemph_or_bold; provide choice on command line for emph/bold using _s
omd_parser.ml: provide choice on command line for emph/bold using _s (github style vs orginal markdown style); cosmetics: command line arguments parsing
- {omd_main.ml,omd_parser.ml}: HTML tags (options on command line+2 sets for inline and block html tags)
- omd_backend.ml: add 'union' to module StringSet