allow writing back to the input file
wip: outputs div

TODO:

1) data uris
2) what does pandoc do with data uris when converting to latex?
3) do we need a pandoc filter to save the uri as file when doing latex?
add data2uri filter
use div for outputs, include all of them
suppress pyout, use data uris, suppress caption
don't quote captions
tidy up
--figures -> --publish