Early LaTeX support

[zazedd]

LaTeX support #425

LaTeX doesn't have code listings by default, so we assume the user has defined an environment for OCaml code and the mdx-errors, each called ocaml and mdx-errors respectively.
For example:

% LSTLISTING

\usepackage{listings}

\lstnewenvironment{ocaml}
  {\lstset{language=[Objective]Caml}}
  {} % styling

\lstnewenvironment{mdx-error}
  {\lstset{language=[Objective]Caml}}
  {} % styling

% MINTED

\usepackage{minted}

\newminted[ocaml]{ocaml}{
  % styling
}

\newminted[mdx-error]{ocaml}{
  % styling
}

Later on we can implement checking for a \lstset or \setminted, and if the language defined inside it is OCaml then instead of \begin{ocaml} for the code, we would accept \begin{lstlisting} or \begin{minted}, so the user doesn't need to as much setup beforehand.

Labels work the same as markdown, just as a comment in LaTeX:

% $MDX file=any_file.txt
\begin{ocaml}
...
\end{ocaml}