My Nix presentations

Create reveal-md shell (FIXME: use nixpkgs-unstable once NixOS/nixpkgs#321453 is merged)
```
nix shell github:imincik/nixpkgs/reveal-md-6.1.2#{reveal-md,chromium}
```

Run presentation

export PRESENTATION_DIR=<DIRECTORY>

reveal-md $PRESENTATION_DIR/presentation.md -w

Export presentation to PDF

reveal-md $PRESENTATION_DIR/presentation.md --print-size A4 --print $PRESENTATION_DIR/presentation.pdf --puppeteer-chromium-executable $(which chromium)