feature request: prepare notebook with specified .py/.ipy file

currently the configuration file is` jupyter_notebook_config.py `in the folder `~/.jupyter`.

i would suggest to provide the freedom of specifying which config file to load when starting Jupiter notebook

for instance, i would like to setup extensions for a project in a specific folder, i can do:
```sh
jupyter notebook --config ./config_for_this_folder.py
```
this will help the Jupiter notebook configuration information to be version controllable and explicable on different PCs / platforms