Add support for <hr> tags in markdown files

[Eakam1007]

For lines in markdown (.md) files which only contain ---, an <hr> tag should be added to the generated html document.

[Eakam1007]

Closed by https://github.com/Eakam1007/rost_gen/commit/f701e109cd3ecf7d34f8f05c78290cb5f266fa55