Add --ipynb-output option

[jgm]

When reading ipynb, currently all output formats from output cells are included. So a standard pandoc conversion will often give both a plain-text fallback and, say, an HTML table, which is usually not desirable.

One fix would be an option --ipynb-output=all|none|best.
all would include all formats, as we do now.
none would omit output cells entirely.
best would select the best format for each output cell, given the destination format.