Feature request: Documents - Copy as markdown for text/plain

Currently, copying a bullet point list from Huly creates a lot of whitespace.

For example:

```
# Title

- one
- two
```

becomes

```
Title

one

two
```

Instead, it should return the original markdown or some other rich text format.

Currently, my solution to getting stuff out is to first export it as JSON, extracting the document with jq, and converting to markdown with [prosemirror](https://github.com/acheong08/pm2md). Not very convenient.