Introduce interpreted text, substitutions and directives into remarkdown

[sgenoud]

The power of docutils (and by extension rst) is in its interpreted text, directives and substitution. Markdown does not have anything comparable (which lead to the many different dialects that exist now).

Also the tools that rely on docutils (i.e. Sphinx) use this mechanism extensively.

remarkdown should extend markdown in order to add these elements to its syntax