Allow for auto-downloading of images referenced with Markdown syntax

[tpwo]

Let's say there is an MD file with an image:

# Some zettel

Lorem ipsum

![](https://example.com/image.png)

After running the proposed feature, code would download the image to disk, save it in a proper folder, and update the Markdown to reference the local image:

# Some zettel

Lorem ipsum

![](./image.png)

I have yet to decide whether images should be kept in zettel folder or in a separate dir like img