Open our output file in append mode always

This avoid overwriting portions of our log output when using Flake8
in verbose mode.

Closes #193

src/flake8/formatting/base.py

@@ -50,7 +50,7 @@ class BaseFormatter(object):
         This defaults to initializing :attr:`output_fd` if :attr:`filename`
         """
         if self.filename:
-            self.output_fd = open(self.filename, 'w')
+            self.output_fd = open(self.filename, 'a')
 
     def handle(self, error):
         """Handle an error reported by Flake8.