xref consumed addendum too early & broke markdown

The Md2Man::Document#reference() method now takes only two parameters:

* input_match - MatchData object for the reference in Markdown input

  * :page - name of the manual page

  * :section - section number of the manual page

* output_match - MatchData object for the reference in output document

  * :addendum - non-space characters immediately
    after the reference in the output document