Enable users who want to track all files processed

This adds two new methods to the BaseFormatter class:

- beginning
- finished

These will indicate when Flake8 begins and finishes processing a file.

Closes #251

tests/integration/test_checker.py

@@ -134,6 +134,7 @@ def test_report_order(results, expected_order):
     file_checker.display_name = 'placeholder'
 
     style_guide = mock.Mock(spec=['options'])
+    style_guide.processing_file = mock.MagicMock()
 
     # Create a placeholder manager without arguments or plugins
     # Just add one custom file checker which just provides the results