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

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

* input_match - MatchData object for the reference in Markdown input

* output_match - MatchData object for the reference in output document
rename md2man(1) to md2man-roff(1); add md2man(5)