Seeding mode

It would be nice to have a command line parameter to run zeronet.py in a mode that it will just download a zite (provided in the command line) and keep it up to date, nothing else. This would be useful to make the zite files available to other apps.
Example: `python zeronet.py --seeding-mode zite_hash`

**Use case:**

This would allow us to run zeronet programmatically to keep a given zite downloaded and up to date so that the json files from that zite can be used by a different script.

Antilibrary.bit has a use case of adding books requested by the users.
When a user requests a new book that is not in the main zite DB, the book ISBN gets added to the user data.json file in the merger db [userdb](http://127.0.0.1:43110/19sZZmJPz1hADwitEs2XT6U46uHLGbHqKi/).
There is a docker container that runs the script that adds the new books. This script will scan the userdb folder for new books requested. Currently I need to have a local instance of zeronet running with the userdb zite added to it, and then I will share the files with the docker container. This requires manual intervention because there is no way to start zeronet in such a way that it will auto-download a zite.
If there was a way to run zeronet in this seeding mode I could automate the deploy of this script to run 2 containers, one with zeronet in the seeding mode for the userdb and the other with the script. No manual intervention would be necessary.

Start in zeronet.py and trace how command-line parameters are parsed and how a zite is downloaded and kept up to date. A complete change would support the --seeding-mode zite_hash entry point, run without other functionality, and maintain the requested zite for external scripts.

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