Subsubsections get numbered in articles

Suppose that there are two LaTeX files `article.tex` and `report.tex`:
```
\documentclass{article}
\begin{document}
  \section{New Section}
  \subsection{New Subsection}
  \subsubsection{New Subsubsection}
\end{document}
```
```
\documentclass{report}
\begin{document}
  \chapter{New Chapter}
  \section{New Section}
  \subsection{New Subsection}
  \subsubsection{New Subsubsection}
\end{document}
```
Currently, `Unite outline` generates outlines for these two files as
```
  1 New Section
    1.1 New Subsection
      New Subsubsection
```
```
  1 New Chapter
    1.1 New Section
      1.1.1 New Subsection
        1.1.1 New Subsubsection
```
respectively. As you can see, in `article.tex` subsubsections don't get numbered, by contrast in `report.tex` subsubsections do get numbered in a weird way (not as 1.1.1.1). This patch changes the outlines as
```
  1 New Section
    1.1 New Subsection
      1.1.1 New Subsubsection
```
```
  1 New Chapter
    1.1 New Section
      1.1.1 New Subsection
        New Subsubsection
```
which resembles the default behaviour of LaTeX and makes more sense.