Add --tag-filter-exclude flag to exclude images, abstract decision logic to interface

[shydefoo]

Summary

• Fixes #65
• Move decision logic to separate interface to allow custom logic to be applied when using gcr-cleaner as a go package.

To delete everything more than 720hours old, except images with tag special-tag

gcr-cleaner-cli -dry-run -repo gcr.io/foo/bar -grace 720h -tag-filter-exclude -tag-filter-any 'special-tag'

[shydefoo]

@sethvargo Requesting review

[sethvargo]

Hi @shydefoo - users should prefer the native Google Artifact Registry functionality instead of gcr-cleaner. We are only fixing bugs and security issues in gcr-cleaner now that there's a native (and free) feature in the Google Cloud product.