[Feature request] Ability to manually purge cached pages/urls

**Is your feature request related to a problem? Please describe.**
According to [this comment](https://github.com/aws-amplify/amplify-console/issues/178#issuecomment-537616693) from AWS it's a *feature* that deleted files don't get purged from the cache until after several more builds. This can present problems for some (possibly even legal ones). I'm opening this up as a feature request, since it seems all the other issues relating to this are getting closed because it's a feature, not a bug.

**Describe the solution you'd like**
Either via UI or through a configuration file/setting/api (preferably all of the aforementioned ways) a way to purge specific urls, or even the entire cache. It could even be a build-time setting. Cloudfront has UI and APIs for this, though not instant, it'd probably be acceptable.

**Additional context**
Related issues: #74, #178, #294

Start by reviewing related issues #74, #178, and #294 plus the linked AWS comment, then compare the CloudFront invalidation UI and API options mentioned in the request. Done means users can purge selected URLs or the entire cache through an agreed supported interface, with behavior documented and verified.

Written by the indexing model from the issue text.