Support for setting options via environment variables

• To persist your choice of not saving sessions for some extended period of time, set an environment variable PYSESSION_SAVE_OFF to True.
  export PYSESSION_SAVE_OFF=True
• To always save your sessions to local file, set an environment variable PYSESSION_SAVE_LOCALLY to True.
  export PYSESSION_SAVE_LOCALLY=True
• The file is saved with a name session.py You can change this by setting the environment variable PYSESSION_FILENAME to your desired filename.
  export PYSESSION_FILENAME=some_file_name.py