Common syntax for passing preprocessor commands

In c we use syntaxes like #Define CONSTNAME 42 to pass preprocessor directives. We should have something similar for preprocessors in general.

There is already pandoc, but having it standardized would make markdown play well with multiple preprocessors besides pandoc. Is there a suitable sintax that could allow for this?

This will be useful for including multiple files. Generating Tables of contents. Filing tables with the latest csv data. Etc…