Native pagebreak support

[kvaleev]

When we remove raw text support in markdown at #25 we need some workaround for widely used newpage command.

Support of multiformat page breaks is on the pandoc roadmap, but the issue is still open: jgm/pandoc#1934

As a workaround, we can consider writing a pandoc filter for the LaTeX page break support (Sidenote: there is a filter for docx page breaks: https://github.com/alexstoick/pandoc-docx-pagebreak.)