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

src/flake8/processor.py

@@ -206,14 +206,19 @@ class FileProcessor(object):
         """Generate the keyword arguments for a list of parameters."""
         if arguments is None:
             arguments = {}
-        for param in parameters:
+        for param, required in parameters.items():
             if param in arguments:
                 continue
             try:
                 arguments[param] = getattr(self, param)
             except AttributeError as exc:
-                LOG.exception(exc)
-                raise
+                if required:
+                    LOG.exception(exc)
+                    raise
+                else:
+                    LOG.warning('Plugin requested optional parameter "%s" '
+                                'but this is not an available parameter.',
+                                param)
         return arguments
 
     def check_physical_error(self, error_code, line):