Incompatibility with `todonotes`

[kprussing]

The current implementation is incompatible with todonotes. It errors when using any of the macros.

MWE:

\documentclass{article}

\usepackage{secretx}
\usepackage{todonotes}

\begin{document}
\chapter*{This}
\unclassified{Test\todo{test}}
\end{document}

Output:

! Undefined control sequence.
l.7 

This needs to be fixed or documented.