- Adapts the template to use Reveal.js and the BUNIGE theme inside
  a parent project with the help of a Makefile. The Pandoc conversion
  command supports the `prefix` variable to specify the path to `dist/`
  or `plugin` when needed.
- Adds in the template the support of another variable (boolean) that
  can be set through the Pandoc command to allow to print the slides in
  a PDF file without the fragments (`-V pdfSeparateFragments=false`).
- Creates some `bunige-` classes to allow to display columns in a slide,
  and to center the columns content vertically, and to remove the figure
  border inside a column, if wanted.
- Update the `bunige-demo.md` file with the new possibilities.
- Update the documentation (`README.md`) accordingly.

Co-Authored-by: iGor milhit <igor@milhit.ch>