Allow using image paths in typst

[mfontanini]

This allows using images in typst by passing in the --root parameter to specify the root path. All images must have an absolute path starting from the directory where the presentation is. So e.g. if you have a layout like

foo/presentation.md
foo/image1.png

In order to use image1.png inside typst in that presentation.md file you need to:

```typst +render
#image("/image1.png")
```

Fixes #233

[chardskarth]

This works for me. Thanks! 🎉.

Should you update the docs as well? Regarding adding images with typst?

[mfontanini]

I tend to only update docs once the change is released, otherwise this will confuse people using the latest released version.