no way to reset static int variable "populated" in mkd_with_html5_tags()

If you call mkd_with_html5_tags() and mkd_deallocate_tags(), calling mkd_with_html5_tags() won't define html5 tags anymore because of a static int variable "populated" in mkd_with_html5_tags(). There could be two ways 1) adding an option to force defining tags (my change does this) or 2) reset "populated" in mkd_deallocate_tags(), which I did not come up with a nice code to do it.