options: Forward --output-file to be reparsed for BaseFormatter

This fixes a regression introduced in daca2c8 and b14d47b.

The --output-file` option was consumed by the preliminary option parser.
However, the `BaseFormatter` class needs the option for setting the
output filename.  This special cases this option to ensure it gets
re-parsed and respected when specified on the CLI.

src/flake8/main/application.py

@@ -112,7 +112,12 @@ class Application(object):
         :rtype:
             (argparse.Namespace, list)
         """
-        return self.prelim_arg_parser.parse_known_args(argv)
+        args, rest = self.prelim_arg_parser.parse_known_args(argv)
+        # XXX (ericvw): Special case "forwarding" the output file option so
+        # that it can be reparsed again for the BaseFormatter.filename.
+        if args.output_file:
+            rest.extend(("--output-file", args.output_file))
+        return args, rest
 
     def exit(self):
         # type: () -> None

tests/integration/test_main.py

@@ -246,3 +246,17 @@ def test_file_not_found(tmpdir, capsys):
     out, err = capsys.readouterr()
     assert out.startswith("i-do-not-exist:0:1: E902")
     assert err == ""
+
+
+def test_output_file(tmpdir, capsys):
+    """Ensure that --output-file is honored."""
+    tmpdir.join('t.py').write('import os\n')
+
+    with tmpdir.as_cwd():
+        _call_main(['t.py', '--output-file=f'], retv=1)
+
+    out, err = capsys.readouterr()
+    assert out == err == ""
+
+    expected = "t.py:1:1: F401 'os' imported but unused\n"
+    assert tmpdir.join('f').read() == expected