Allow setting fields in Tana Intermediate Format

For import into Tana it would be great if we could use some of the available attributes to set field values.

For example, if I write this as a template right now
```
>Date:: {created-at-block} {created-at} {end-created-at-block}
>URL:: {source-url-block} {source-url} {end-source-url-block}
>Topic(s):: {tags-block} {tags} {end-tags-block}

{content-block}{content}{end-content-block}
```

Tana imports this like so:

Instead, it would be great if Tana would show it like this: