Bug: Code comments are shifted as they where headings

[hsjobeki]

Currently the headings shifting is done by line. Not with an actual markdown parser.

# Heading

```nix
# Comment
```

Will result in

### Heading

```nix
### Comment
```

Notice that the code comment got shifted as well. To fix this we could use a markdown parser, or use a simple regex to check if we are within a code block.