[feature] Add custom environment to be indented

[vepain]

As .latexindent.yaml config file enables adding a custom environment to indent, it would be great to have a similar feature here.

For example, my custom env labeling which is a wrapper of description is not currentrly indented:

\begin{labeling}
\item[label] text
\end{labeling}

Wanted output:

\begin{labeling}
  \item[label] text
\end{labeling}

[WGUNDERWOOD]

Thanks, this is a good idea. I think I will try to get the configuration file functionality implemented first (#39), as passing custom environments via CLI arguments will be unwieldy!