include cell outputs in markdown output

enable with `--with_outputs` switch

e.g.

    notedown r-examples/r-example.ipynb --reverse
    notedown r-examples/r-example.ipynb --reverse --with_outputs
implement pandoc attributes and reading inputs

Now we can read outputs that have been written to markdown, as long as
they use a special syntax. The input / output cells are represented by
two consecutive code blocks, the second of which must have pandoc
attributes containing the classes `.json` and `.output`. i.e. like this:

```{.python .input}
code
```

```{.json .output}
outputs
```

This is enabled by allowing pandoc attributes on code blocks.