Add date_format config key for csv command

- Add get_date_format() to config.py: reads [csv] date_format, returns
  None when absent (falls back to the default %d/%m/%y)
- _cmd_csv applies the format to both single-day and weekly date strings
- Add 3 tests for get_date_format in TestGetters
- Update timesheets.example.toml and README with [csv] date_format key

README.md

@@ -100,6 +100,9 @@ map = "/path/to/project_map.json"
 [work]
 daily_hours = 8.0
 weekly_hours = 40.0
+
+[csv]
+date_format = "%d/%m/%y"  # default; any strftime format string
 ```
 
 Priority order: **CLI flag > config file > environment variable > default**.