Feature request: passing lists in `[extra]`

I like something like this:

```
[group.activities]
paths=["activities"]

[group.activities.note]
filename = "{{id}}-{{slug title}}"
extension = "md"
template="activities.md"

[group.activities.extra]
author = "Me"
keywords = ["activities", "really stuff"]
```

and in template:

```
keywords: {{group.extra keywords]]
```

But it gives me an error