Added link extension method to markup generators

Markdown files on github wikis don't use the extension.
Added Gossipdoc>>#new:

It takes a Markup generator. By default, HTMLMarkup is used.
Added generatorClass instvar to Gossipdoc
Changed Gossipdoc class comment
Added class comment and variables headers to Gossipdoc output
Added class comment to Gossipdoc output
Added class variables to Gossipdoc output
Added instance variables to Gossipdoc output
Minor bugfix to HTMLMarkup>>#heading:level:
Removed extraneous code in MarkdownMarkup>>#heading:level:
Gossipdoc: Only convert whitespaces when writing HTML
Convert whitespaces when writing HTML

There are better ways to do whis
Avoid attempting to send #do: to nil objects in Gossipdoc
Added accessors for Gossipdoc instvar generatorClass
Extracted method Gossipdoc>>#writeVariablesForClass:using:

#createForClass: was getting too big