Support exactly one cell output

[kokes]

As discovered in #28, we sometimes render multiple cell outputs, but we should always render just one. There is an ordering defined in nbconvert, so we should check each when parsing the data field.

Before implementing this

1. let's check a few notebooks (from tests, see Build regression tests and a harness #33) and how they render before and after.
2. see if we can implement some of the mime types that are not yet supported (e.g. application/pdf?)
3. grep as many notebooks as possible and check their mime types - maybe there our some extra mime types out there

[kokes]

We're now rendering at most one cell output (255fc3e) and I've checked it all against the tests in nbformat and nbconvert, but I have yet to resolve the other points in this issue.