Checkboxes: Grey out completed tasks

```
- [ ] This is an action item.
- [x] This one is completed. I want this line to be gray.
```

Is it possible to gray out the second line? It would make my TODO-lists much more readable.

I tried to add a keyword `- [x]` in the UDL-form, but I didn't figure out how to do it right. The space is interpreted as a seperator. Quote marks, parentheses, or`-\s[x]` don't do the trick either.