First alpha! - The save path is now a constant

• In #12, stopped storing the benchmark results in a file named after the current date and time.
  Instead, it will always save into .django-queries and won't contain a json file extension
  anymore to make it less appealing as it's not meant to be read by a human.
• In #12, dropped the environment variable PYTEST_QUERIES_SAVE_PATH and replaced
  and introduced the --django-db-bench PATH option instead, which does exactly the same thing.

TL;DR: a report will be generated in your current working directory and will be called .pytest-queries.
To override the save path, pass the --django-db-bench PATH option to pytest.