Add option to not increase depth in certain environments

[tdegeus]

In particular

... 

\begin{appendices}

    ...

\end{appendices}

could be annoying if indented

[tdegeus]

The biggest question is what the syntax should be. Clarifying it could improve the implicit distinction made for \begin{document} that is also not indented.

The current mechanism of

% \begin{texindent}{indent=""}
\begin{appendices}

\end{appendices}
% \end{texindent}

is clearly not what you want, as it would remove all indentation witting the appendices. Instead, the custom indentation should just apply to the indentation after \begin{appendices}