Add a flag for exiting with status code 0, even when there are warnings

This might be useful e.g. in a build system where exiting with status code > 0
always means the build failed. In that case, you still want to signify that
flake8 itself ran without problems, even when there are warnings.

flake8/run.py

@@ -59,6 +59,9 @@ def main():
     else:
         stdin = sys.stdin.read()
         warnings += check_code(stdin, complexity)
+
+    if options.exit_zero:
+        raise SystemExit(0)
     raise SystemExit(warnings > 0)