Changed the report save path to be always the same one & drop env for it

[NyanKiyoshi]

Closes #10.

• In Generate sums of queries #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 Generate sums of queries #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.

Changes Checklist

• The changes were tested (manually)
• The changes are tested automatically (pytest)
• The changes are optimized and clean
• The changes are documented:
  • The code is documented
  • The readme is up to date
  • The documentation (readthedocs) is up to date and tested

[codecov-io]

Codecov Report

Merging #13 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #13   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         130    128    -2     
  Branches       13     13           
=====================================
- Hits          130    128    -2

Impacted Files	Coverage Δ
pytest_django_queries/plugin.py	100% <100%> (ø)	⬆️

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