Add option to bulk delete images

[nightfurys]

Starting anchore-engine v0.8.0 API support for bulk deletion of images is available. Extend anchore-cli image deletion functionality to accept multiple image artifacts(digests/tags/image ids). Additional response handling required for single and bulk deletion since image cleanup process on the backend is entirely async. May need to keep the current response handler for backwards compatibility for a few releases

[nightfurys]

Consumes https://github.com/anchore/anchore-engine/issues/502

[nightfurys]

PR needs rebase and review. Targeting it for next release