Allow starting the server with both file_to_run and notebook_dir...
Allow starting the server with both file_to_run and notebook_dir
file_to_run and notebook_dir would collide, with possible dictionary
ordering randomness depending on how they were passed.
With this change, the default value of notebook_dir is pulled from
file_to_run, instead of pushing the value when file_to_run is changed.
This makes it possible to specify both and have the server behave as
expected.