diff --git a/flake8/main.py b/flake8/main.py
index e1d65c5..297618a 100644
--- a/flake8/main.py
+++ b/flake8/main.py
@@ -6,7 +6,7 @@ import sys
 import setuptools
 
 from flake8.engine import get_parser, get_style_guide
-from flake8.util import is_flag, flag_on
+from flake8.util import option_normalizer
 
 if sys.platform.startswith('win'):
     DEFAULT_CONFIG = os.path.expanduser(r'~\.flake8')
@@ -104,8 +104,7 @@ class Flake8Command(setuptools.Command):
             value = getattr(self, option_name)
             if value is None:
                 continue
-            if is_flag(value):
-                value = flag_on(value)
+            value = option_normalizer(value)
             # Check if there's any values that need to be fixed.
             if option_name == "include" and isinstance(value, str):
                 value = re.findall('[^,;\s]+', value)
diff --git a/flake8/tests/test_util.py b/flake8/tests/test_util.py
new file mode 100644
index 0000000..32a1d44
--- /dev/null
+++ b/flake8/tests/test_util.py
@@ -0,0 +1,86 @@
+import unittest
+
+from flake8.util import option_normalizer
+
+
+class TestOptionSerializer(unittest.TestCase):
+
+    def test_1_is_true(self):
+        option = option_normalizer('1')
+        self.assertTrue(option)
+
+    def test_T_is_true(self):
+        option = option_normalizer('T')
+        self.assertTrue(option)
+
+    def test_TRUE_is_true(self):
+        option = option_normalizer('TRUE')
+        self.assertTrue(option, True)
+
+    def test_ON_is_true(self):
+        option = option_normalizer('ON')
+        self.assertTrue(option)
+
+    def test_t_is_true(self):
+        option = option_normalizer('t')
+        self.assertTrue(option)
+
+    def test_true_is_true(self):
+        option = option_normalizer('true')
+        self.assertTrue(option)
+
+    def test_on_is_true(self):
+        option = option_normalizer('on')
+        self.assertTrue(option)
+
+    def test_0_is_false(self):
+        option = option_normalizer('0')
+        self.assertFalse(option)
+
+    def test_F_is_false(self):
+        option = option_normalizer('F')
+        self.assertFalse(option)
+
+    def test_FALSE_is_false(self):
+        option = option_normalizer('FALSE')
+        self.assertFalse(option)
+
+    def test_OFF_is_false(self):
+        option = option_normalizer('OFF')
+        self.assertFalse(option)
+
+    def test_f_is_false(self):
+        option = option_normalizer('f')
+        self.assertFalse(option)
+
+    def test_false_is_false(self):
+        option = option_normalizer('false')
+        self.assertFalse(option)
+
+    def test_off_is_false(self):
+        option = option_normalizer('off')
+        self.assertFalse(option)
+
+    def test_parses_lists(self):
+        answer = ['F401', 'F402', 'F403', 'F404']
+
+        option = option_normalizer('F401,F402,F403,F404')
+        self.assertEqual(option, answer)
+
+        option = option_normalizer('F401 ,F402 ,F403 ,F404')
+        self.assertEqual(option, answer)
+
+        option = option_normalizer('F401, F402, F403, F404')
+        self.assertEqual(option, answer)
+
+        option = option_normalizer('''\
+            F401,
+            F402,
+            F403,
+            F404,
+        ''')
+        self.assertEqual(option, answer)
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/flake8/util.py b/flake8/util.py
index fda6331..da33f42 100644
--- a/flake8/util.py
+++ b/flake8/util.py
@@ -38,12 +38,6 @@ class OrderedSet(list):
             self.append(value)
 
 
-def is_flag(val):
-    """Guess if the value could be an on/off toggle"""
-    val = str(val)
-    return val.upper() in ('1', '0', 'F', 'T', 'TRUE', 'FALSE', 'ON', 'OFF')
-
-
 def is_windows():
     """Determine if the system is Windows."""
     return os.name == 'nt'
@@ -63,6 +57,13 @@ def force_disable_jobs(styleguide):
     return is_windows() or is_using_stdin(styleguide.paths)
 
 
-def flag_on(val):
-    """Return true if flag is on"""
-    return str(val).upper() in ('1', 'T', 'TRUE', 'ON')
+def option_normalizer(value):
+    if str(value).upper() in ('1', 'T', 'TRUE', 'ON'):
+        value = True
+    if str(value).upper() in ('0', 'F', 'FALSE', 'OFF'):
+        value = False
+
+    if isinstance(value, str):
+        value = [opt.strip() for opt in value.split(',') if opt.strip()]
+
+    return value