--offline flag

[terriko]

• From our refactoring wishlist Refactoring wishlist (Meeting notes 2021-07-28) #1291

We currently have an offline guide here: https://cve-bin-tool.readthedocs.io/en/latest/how_to_guides/offline.html

It might be nice to have an --offline flag that was shorthand for –update never and –disable-version-check and have it change a few messages to be more applicable to offline users. These might include:

1. If the DB doesn't exist, print a link to the offline guide and exit.
2. If the DB is old, print the age of the data and a link to the offline guide so people are reminded how to update the data (but don't stop execution)