0.20.0: Allow protecting images with specified tags (#56)

what

• Allow protecting images with a given set of tag names

why

• At Transcend, we tag images with dev, staging, and prod for deployments in addition to their SHA tags. We want to expire images, but not those actively is use.

references

• aws/containers-roadmap#637
• https://docs.aws.amazon.com/AmazonECR/latest/userguide/lifecycle_policy_examples.html

Note: This change is fully backwards compatible