move Md2Man::Engine into Md2Man::Roff namespace
stop cross-reference expansion in HTML directives

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
rename md2man(1) to md2man-roff(1); add md2man(5)
rename "manpage-reference" class to "md2man-xref"
move stuff out of README and into manual pages