Handle optional parameters that were never supported

Previously, pycodestyle never introspected the argument names for
classes except to require that ``tree`` be an argument it could pass.
For Flake8 3.0, we lifted that restriction, but old plugins seem to
have cargo-culted their __init__ signature to be

    def __init__(self, tree, builtins=None):

For some yet unknown reason. This was causing an AttributeError. By
updating flake8.utils.parameters_for to return a dictionary that
indicates whether the parameter is required or not, we can side-step
this by simply ignoring the parameter if it has a default value and
we cannot provide it.

Closes #151

tests/unit/test_file_processor.py

@@ -114,13 +114,17 @@ def test_check_physical_error(error_code, line, expected_indent_char):
 
 
 @pytest.mark.parametrize('params, args, expected_kwargs', [
-    (['blank_before', 'blank_lines'], None, {'blank_before': 0,
-                                             'blank_lines': 0}),
-    (['noqa', 'fake'], {'fake': 'foo'}, {'noqa': False, 'fake': 'foo'}),
-    (['blank_before', 'blank_lines', 'noqa'],
+    ({'blank_before': True, 'blank_lines': True},
+        None,
+        {'blank_before': 0, 'blank_lines': 0}),
+    ({'noqa': True, 'fake': True},
+        {'fake': 'foo'},
+        {'noqa': False, 'fake': 'foo'}),
+    ({'blank_before': True, 'blank_lines': True, 'noqa': True},
         {'blank_before': 10, 'blank_lines': 5, 'noqa': True},
         {'blank_before': 10, 'blank_lines': 5, 'noqa': True}),
-    ([], {'fake': 'foo'}, {'fake': 'foo'}),
+    ({}, {'fake': 'foo'}, {'fake': 'foo'}),
+    ({'non-existent': False}, {'fake': 'foo'}, {'fake': 'foo'}),
 ])
 def test_keyword_arguments_for(params, args, expected_kwargs):
     """Verify the keyword args are generated properly."""

tests/unit/test_utils.py

@@ -102,17 +102,22 @@ def test_parameters_for_class_plugin():
 
     plugin = plugin_manager.Plugin('plugin-name', object())
     plugin._plugin = FakeCheck
-    assert utils.parameters_for(plugin) == ['tree']
+    assert utils.parameters_for(plugin) == {'tree': True}
 
 
 def test_parameters_for_function_plugin():
     """Verify that we retrieve the parameters for a function plugin."""
-    def fake_plugin(physical_line, self, tree):
+    def fake_plugin(physical_line, self, tree, optional=None):
         pass
 
     plugin = plugin_manager.Plugin('plugin-name', object())
     plugin._plugin = fake_plugin
-    assert utils.parameters_for(plugin) == ['physical_line', 'self', 'tree']
+    assert utils.parameters_for(plugin) == {
+        'physical_line': True,
+        'self': True,
+        'tree': True,
+        'optional': False,
+    }
 
 
 def read_diff_file(filename):