Formatters should support multiple MIME types as a standard behavior

[jonsequitur]

Currently, output typically only contains a single FormattedValue, though you can provide more than one by creating a DisplayEvent directly.

There should be a way equivalent to Formatter.SetPreferredMimeTypeFor(Type type, string preferredMimeType) that tells the display methods to generate FormattedValue instances for multiple MIME types at one time. The .ipynb format allows for storing multiple outputs of differing MIME types and this can help provide options for rendering and publishing, for example when displaying the notebook in the absence of a running kernel.