skaffold should offer a `clean`

Certain builders create files in the local workspace. The `jib` builder, for example, invokes Maven or Gradle which create files in a `target` or `build` directory (respectively). The `bazel` builder creates special symlinks in the root of the workspace, and it's possible that `custom` builders might also create files or directories.

It should be possible to run `skaffold clean` to ensure the entire workspace is clean. We could also include a `prune` option to remove cached or built images too.