Use the first reference found.

Don't overwrite references with later ones, this comes from CommonMark.
Also disable AUTO_LINK in mmark cmd so we can actually pass some
more tests
Allow for spaces after atx header
atx header directly after name is allowed.

 # foo# should have the header name 'foo#', this is from CommonMark.
 We now have 220 failing tests from CommonMark (from 524 total) which
 is 58%.
emphasis preceed by alnum and follow by punct is not emphasis
Do emphasize it is followed by *
Updates to the readme
Update doc text