Provide viur-cli export tool

[phorward]

viur-cli export command and sub-commands, providing general export tooling for ViUR databases, kinds or specific entities, with the following features:

• Export as Google Datastore Backup
• Export to JSON
• Export to CSV

For the tools especially for JSON and CSV export should, it should be considered to use the MEX language integrated into a Mausbrand customer project as domain specific language to describe tabular output formats with conditional output and other tools.

[Grashalmbeisser]

Is this feature still necessary, i think the viur script0r is able to handle these requirements aswell.

[phorward]

IMHO it is necessary. The vision is: Dump entire database into a file, or fill entire database from a file. <3 Think about the possibilities, even for spreading VIUR.

For sure, it might be interesting to have a scriptor import/export library which is simply used to do this, and integrated into viur-cli.