Add support for bucket object --folder option.

[markeissler]

This PR adds support for specifying a bucket object folder via a new --folder option. This update is meant to be backwards compatible with the current --target option and will append a path to any object path discovered from the initial --target option value.

The motivation for this change is that I have a drone workflow wherein the bucket value is retrieved from a drone secret (using from_secret). I need to dynamically create the object name folder path during CI. By separating the bucket value from the folder path it makes this workflow possible.

Both the README and tests have been updated in support of this PR.

[markeissler]

CI appears to be failing because of this error in dry-run:

3.12: Pulling from library/alpine
11s
109	toomanyrequests: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit
11s
110	time="2021-07-10T23:34:49Z" level=fatal msg="exit status 1"

Not sure how to deal with the pull rate limit.

[markeissler]

Bouncing this because there's no way to re-start the test.

[lafriks]

restarted