json rendering support

JsonRenderer will create changelog.json.  I've updated the README.md to reflect the new functionality.  I also made some preliminary commits to pull up some functionality from SimpleHtmlRenderer to its parent FileRenderer so I didn't have to duplicate common logic between SimpleHtmlRenderer  and JsonRenderer.

I have generateJSONChangeLog true by default, that isn't required.  So if you think the json file shouldn't be written by default, then no argument from me.