parse nroff-formatted manpages

 This abstract subclass of Parser::MGC recognises nroff grammar from a file or
 string value. It invokes methods when various nroff directives are
 encountered. It is intended that this class be used as a base class, with
 methods provided to handle the various directives and formatting options.
 Typically a subclass will store intermediate results in a data structure,
 building it as directed by these method invocations.