tmdd obj
_new in XS and add html5 and gfm flags
call mkd_with_html5_tags() if html5
13.html5.t
remove gfm feature. There are very few differences from `mkd_string` and `gfm_string`.
add document of html5
update MANIFEST
update document
call `mkd_deallocate_tags` if html5 flag is off"