Add --raw flag to csv command to skip aggregation

- Add `to_csv_entries()` to output.py: converts raw rows to write_csv
  entries one-for-one, without merging by (project, description)
- Add `--raw` flag to the csv subparser; _cmd_csv branches on it
- Add TestToCsvEntries with 6 tests
- Update README with --raw usage example
- Add .coverage and htmlcov/ to .gitignore

.gitignore

@@ -13,3 +13,7 @@ wheels/
 timesheets.toml
 
 timesheets.csv
+
+# Test coverage
+.coverage
+htmlcov/