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

src/flake8/checker.py

@@ -260,8 +260,9 @@ class Manager(object):
         results_reported = results_found = 0
         for checker in self.checkers:
             results = sorted(checker.results, key=lambda tup: (tup[1], tup[2]))
-            results_reported += self._handle_results(checker.display_name,
-                                                     results)
+            filename = checker.display_name
+            with self.style_guide.processing_file(filename):
+                results_reported += self._handle_results(filename, results)
             results_found += len(results)
         return (results_found, results_reported)
 

src/flake8/formatting/base.py

@@ -44,6 +44,24 @@ class BaseFormatter(object):
         """Initialize the formatter further."""
         pass
 
+    def beginning(self, filename):
+        """Notify the formatter that we're starting to process a file.
+
+        :param str filename:
+            The name of the file that Flake8 is beginning to report results
+            from.
+        """
+        pass
+
+    def finished(self, filename):
+        """Notify the formatter that we've finished processing a file.
+
+        :param str filename:
+            The name of the file that Flake8 has finished reporting results
+            from.
+        """
+        pass
+
     def start(self):
         """Prepare the formatter to receive input.
 

src/flake8/style_guide.py

@@ -1,5 +1,6 @@
 """Implementation of the StyleGuide used by Flake8."""
 import collections
+import contextlib
 import enum
 import linecache
 import logging
@@ -120,6 +121,13 @@ class StyleGuide(object):
 
         return Selected.Implicitly
 
+    @contextlib.contextmanager
+    def processing_file(self, filename):
+        """Record the fact that we're processing the file's results."""
+        self.formatter.beginning(filename)
+        yield self
+        self.formatter.finished(filename)
+
     def _decision_for(self, code):
         # type: (Error) -> Decision
         select = find_first_match(code, self._all_selected)