Fix compatibility with flake8 3.5

Merge jezdez/pytest-flake8 to fix compatibility with flake8 3.5.0 option
handling.

Fixes #34

pytest_flake8.py

@@ -178,6 +178,8 @@ def check_file(path, flake8ignore, maxlength, maxcomplexity,
     if statistics:
         args += ['--statistics']
     app = application.Application()
+    app.parse_preliminary_options_and_args(args)
+    app.make_config_finder()
     app.find_plugins()
     app.register_plugin_options()
     app.parse_configuration_and_cli(args)