Feature(MD): Add commandline option to pass parameters to renderers.

  -- currently implemented only for the LaTeX renderer.
Feature(MD): Wrap sections (i.e. started by <h.> tags) into <section> tags.
Feature(MD): Include referenced elements in filtered view.

  -- note: part of this already in the previous commit which actually calls
     the code is present.
BugFix: Check that md has the Meta attribute before accessing it.
Feature(LATEX): Allow unnumbered sections.
Misc(LATEX): Document the fact that <a> handling is not perfect.
Feature(LATEX): Optionally allow embedding of linked files into the pdf.