Wire-up --statistics again

I'm not sure where this code went or when, but it disappeared. Let's
add it back.

Related #180

src/flake8/formatting/base.py

@@ -85,6 +85,19 @@ class BaseFormatter(object):
         raise NotImplementedError('Subclass of BaseFormatter did not implement'
                                   ' format.')
 
+    def show_statistics(self, statistics):
+        """Format and print the statistics."""
+        for error_code in statistics.error_codes():
+            stats_for_error_code = statistics.statistics_for(error_code)
+            statistic = next(stats_for_error_code)
+            count = statistic.count
+            count += sum(stat.count for stat in stats_for_error_code)
+            self._write('{count:<5} {error_code} {message}'.format(
+                count=count,
+                error_code=error_code,
+                message=statistic.message,
+            ))
+
     def show_benchmarks(self, benchmarks):
         """Format and print the benchmarks."""
         # NOTE(sigmavirus24): The format strings are a little confusing, even

src/flake8/main/application.py

@@ -263,6 +263,13 @@ class Application(object):
         LOG.info('Found a total of %d violations and reported %d',
                  self.total_result_count, self.result_count)
 
+    def report_statistics(self):
+        """Aggregate and report statistics from this run."""
+        if not self.options.statistics:
+            return
+
+        self.formatter.show_statistics(self.guide.stats)
+
     def initialize(self, argv):
         # type: () -> NoneType
         """Initialize the application to be run.
@@ -285,6 +292,7 @@ class Application(object):
         self.initialize(argv)
         self.run_checks()
         self.report_errors()
+        self.report_statistics()
         self.report_benchmarks()
 
     def run(self, argv=None):

src/flake8/statistics.py

@@ -9,6 +9,16 @@ class Statistics(object):
         """Initialize the underlying dictionary for our statistics."""
         self._store = {}
 
+    def error_codes(self):
+        """Return all unique error codes stored.
+
+        :returns:
+            Sorted list of error codes.
+        :rtype:
+            list(str)
+        """
+        return list(sorted(set(key.code for key in self._store.keys())))
+
     def record(self, error):
         """Add the fact that the error was seen in the file.