Add support for importing baselines

[domanchi]

Summary

Now that we have the capability to label secrets in the baseline, we need to address the issue of how we treat version bumps with potentially labelled data.

For any version before 0.9, our main strategy is to just ask the user to recreate the baseline. However, versions after 0.9 may have labelled data, and merely recreating the baseline would potentially eliminate all labelled data. To support this, we introduce the concept of importing baselines.

Testing

detect-secrets$ detect-secrets --scan test_data > .secrets.baseline
detect-secrets$ detect-secrets --audit .secrets.baseline

<auditing-happens>

detect-secrets$ detect-secrets --scan test_data < .secrets.baseline > .secrets.baseline.new
detect-secrets$ diff .secrets.baseline .secrets.baseline.new
3c3
<   "generated_at": "2018-06-27T19:50:04Z",
---
>   "generated_at": "2018-06-27T20:07:19Z",
104c104

Another way of importing it is by using the --import flag.

detect-secrets$ detect-secrets --scan test_data --import .secrets.baseline > .secrets.baseline.new

[KevinHock]

LGTM

[KevinHock]

TIL 👍