new: dev: separated creation of the data structure and its rendering.

This commit prepares for any templating system to take back the job of rendering.
fix: random cosmetics changes @minor
new: added templating system and examples with ``mustache`` template support for restructured text and markdown output format.
new: doc: added some information on path lookup scheme to find ``gitchangelog.rc`` configuration file.
new: added a ``mako`` output engine with standard ReSTructured text format for reference.