feat: display section number of LaTeX section headings

[sankantsu]

Overview

Thank you for awesome plugin!
I love using it!

This PR improves display of LaTeX sections.
Here is screenshots which describs the change of results window display.

• Original

• Display with section number (which this PR implements)

Some background

Usually LaTeX files are finally rendered into pdf file, and section numbers (which are automatically caluculated) are displayed in resulting pdf files.

Here is an example.

• LaTeX source (test-heading.tex)

\documentclass{article}

\begin{document}

\section{Section}
\subsection{Subsection}
\subsubsection{Subsubsection}
\section{Section2}
\subsection{Subsection}
\subsubsection{Subsubsection}
\subsubsection{Subsubsection2}
\subsection{Subsection2}
\subsubsection{Subsubsection}
\subsubsection{Subsubsection2}

\end{document}

• Rendered pdf (produced by pdflatex test-heading.tex)

What this PR implements

I think it is nice to see section numbers also in picker results window, rather than displaying raw LaTeX command (like \subsection{...}.
So I implemented the entry display with section numbers.

I added picker (opt-in) option use_section_number for the main picker, and if it is set true use display with section numbers.
I also added description of this option in README.

Caveats

This display style assumes that the document have *article documentstyle.
*article type documentstyle is most common, but for other document styles picker display and rendered display can differ.