Add --offline option

[jayfk]

safety-db is now installable via pip, add it as an option.

[efokschaner]

I'd like to +1 this suggestion, and ask for a built-in option to use safety-db, as it allows external sources of build failure to be managed with dependabot like other external dependencies changing, rather than suddenly breaking a codebase which previously passed tests (although I understand that can be considered a feature in vulnerability scanning).