Refactor test_should_output_csv_format_for_files
Refactor splitting core.py into model files
Refactor fix PEP8 violations in step definitions
Fix bug in to_csv (Use file handler, not filename)
Finish `Given` step "I have COA.."

This step is now in Green phase (of Red -> Green -> Refactor)

```
  Scenario Outline: Generating General Ledger file format          # features/command_line_ledger.feature:6
    Given I have a Chart of Accounts with '1010' and 'Petty Cash'  # features/steps/command_line_ledger.py:7 0.002s
```