Some updates related to company-mode integration

See comments at the bottom here for justification: https://github.com/company-mode/company-mode/issues/31

While testing the result, I noticed that this actually removes a bit of functionality that someone might find useful: the preview for single completion doesn't show its type anymore. If it's indeed a loss, feel free to file a feature request. Displaying annotation in `company-preview-frontend` might be useful for other backends, too.

/cc @bbbscarter