Replace line breaks in headers in the TOC

This is an improved patch. I have created a new topic branch for the changes.

Most headers do not contain line breaks in the TOC, therefore Pandoc will remove line breaks in headers in the TOC by default. If the header has the class `unnumbered` then the line breaks in the TOC won't be removed.

Only the LaTeX and HTML writers are supported at this time.

Markdown:
```
## JSON\
filters
```

LaTeX:
```latex
\subsection[JSON filters]{JSON\\filters}\label{json-filters}
```