License should be configurable in the TTL output. Currently even worse than this it is defaulting to CC0 even if your data isn't.
Users need this because they have data that isn't CC0 that the don't want to be falsely saying it is
-----
https://www.wikidata.org/wiki/Special:EntityData/Q8743220.ttl
has
```
@prefix cc: <http://creativecommons.org/ns#> .
cc:license <http://creativecommons.org/publicdomain/zero/1.0/> ;
```
for example
Ideally this should probably be configured the same way as the license visible on the Wiki.
Having joint configuration for this licence and other licences within Wikibase is not covered in this ticket.
**Acceptance Criteria**
[] The licence displayed in the .ttl output is configurable
[] The default is CC0 / the current hardcoded licence
[] The option is documented in the docs/options.wiki file