Integration of the extension impress.js-progress

Love hovercraft simplicity but absolutely need numbered slides for scientific/academic presentations.
I found  the ``impress.js-progress`` extension from m42e and then added its files into ``default/template.cfg`` and ``simple/template.cfg``.
Then added two simple <xsl:test> in ``template.xsl`` in order to add one pair of new parameters in .rst files: ``progressbar`` and ``progress``.
Eventually added one sentence in the help file.
Still very basic and should be improved to be more versatile.