read stdin, write stdout. fails tests.

usage:

    cat blah.ipynb | runipy > output.ipynb

why? so that you can pipeline with notedown:

    notedown blah.md | runipy > blah.ipynb

needs to cope with optionally specifying files in the arguments.

could raise an error if using stdin and input_file / output_file are
specified in args?

could use '--' or similar to indicate output to be stdout or input to be
stdin.

todo: let notebook runner take a string as the file argument instead of
a file object - to preserve original behaviour.