Provide viur-cli import tool

[phorward]

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

• Import from Google Datastore Backup
• Import from JSON
• Import from CSV (see Integrate tools from viur-tools into viur-cli #3, csvimport.py)

The features from csvimport.py are necessary, e.g. looking up for keys or codes, replacing values etc. (logics-expressions).

[phorward]

As commented in #5 (comment), this feature would be very useful and should be done using viur-scriptor, but be part of viur-cli.