Improve default human-friendly output.

Since most packages are under vcs, display only those that aren't with `?` symbol, instead of using `@` to indicate presence of vcs.

I've been testing this change locally for the last week, and after (quickly) getting used to it, it feels like a net positive change. Being under a vcs repo makes sense as the default. It makes the output cleaner, see Sample Output.

The `--plumbing` presenter remains unchanged.

This PR also adds a DebugPresenter, available via --debug flag. It's meant to be used for debugging gostatus itself, and may be removed eventually once it's not needed.