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

tests/test_config.py

@@ -6,6 +6,7 @@ import pytest
 from timesheets.config import (
     DEFAULT_CONFIG_FILENAME,
     find_default_config,
+    get_date_format,
     get_map_path,
     get_token,
     load_config,
@@ -86,3 +87,12 @@ class TestGetters:
 
     def test_get_map_path_missing_key(self):
         assert get_map_path({"projects": {}}) is None
+
+    def test_get_date_format_present(self):
+        assert get_date_format({"csv": {"date_format": "%Y-%m-%d"}}) == "%Y-%m-%d"
+
+    def test_get_date_format_missing_section(self):
+        assert get_date_format({}) is None
+
+    def test_get_date_format_missing_key(self):
+        assert get_date_format({"csv": {}}) is None