Add update-reference and move-reference

[VojtechDostal]

At https://www.wikidata.org/wiki/Wikidata:Bot_requests, the most frequent requests tend to be those involving changes in references. This is because there is currently no tool which can update reference property values or change properties.

It would be great if Wikibase-cli had these built-in functionalities :

• update-reference = change value of a reference property
• move-reference = change property of a reference