New templating work-around to build HTML and RSS files

Short how-to:
- the templates are stored and declared in the `publish-templates` script
- the global templates (whole index.html and index.rss) are parsed replacing
  any '%TAGNAME%' string by its equivalent (see the comments in the file)
- the templates will be merged using local version preferably
- the `publish_achievements` method is updated to use the templates

Note: the templates used to build the list items of unlocked and locked
achievements are used with the internal `sprintf` method ; a classic usage
could be: `TPL="<li>title: %s ; content: %s</li>"`
adding the 'style.css' file in .gitattributes to keep local changes preferably