Add ability to use a pre-cached vulnerability db

[zhill]

Users should be able to skip the init/download step if they manually setup the db in the proper location. This is useful for CI/CD use-cases where cross-job caches exist.

[wagoodman]

@zach, coming back to this ticket after a while... there are a few features that I think capture this already by this point:

• config option that enables/disables the auto-update on startup (option can also be set via env var)
• db import command that allows someone to provide the path of a vulnscan-db archive already on disk and import it for general use.
• since we're using go-getter for db archive downloading and importing, in theory we can allow a user to specify sources from git, http, s3, and gcp buckets instead of just local disk... I haven't tried it explicitly yet, but I think we're primed for it.

Mind if I close this?

[zhill]

Yep, lgtm.

go-getter is a nice addition here, those are solid options.