Support for the k notation in the \num command

[maxfrei750]

First: thanks for the awesome work you're doing.

I have a feature request:

In some fields of science (e.g. computer science, e.g. this paper) large numbers without units are often abbreviated using the kilo prefix.

Examples:

After 21k iterations, the loop was terminated.

There are possible workarounds:

\documentclass{article}
\usepackage{siunitx}
\DeclareSIUnit\k{k}
\begin{document}
After \SI{21}{\k} iterations, the loop was terminated
\end{document}

\documentclass{article}
\usepackage{siunitx}
\begin{document}
After \SI{21}{\kilo{}} iterations, the loop was terminated.
\end{document}

However, both seem hacky and not very elegant.

I would much more prefer a solution like:
\num{21k} or \num[k-notation = true]{21000}

Thanks.