Suggestion: Show public images by default

Hi, I think the required `--public` flag for `doctl image list` is a bit counterintuitive. It seems to have tripped up others in the past and this was changed for `list-distribution` and `list-application` in #280.

I think it is a better UX for a list command to list all resources and then you should opt in to filtering them if you want. This would also match the behavior of `list-distribution` and `list-application`. Moreover, newcomers to the CLI are probably using `image list` to find an image slug to use in other commands, so making `--public=true` the default will help this beginner use case.