mdx enables execution of code blocks inside markdown files. There are
(currently) two sub-commands, corresponding to two modes of operation:
preprocessing (`ocaml-mdx pp`) and tests (`ocaml-mdx test`).

The preprocessor mode enables mixing documentation and code, and the
practice of "literate programming" using markdown and OCaml.

The test mode enables ensuring that shell scripts and OCaml fragments in
the documentation always stay up-to-date.

The blocks in markdown files can be parameterized by `mdx`-specific
labels, that will change the way `mdx` interprets the block. The syntax
is: `<!-- $MDX labels -->`, where `labels` is a list of valid labels
separated by a comma. This line must immediately precede the block it
is attached to.