Do not override options unneccesarily

Merge options loaded instead of replacing them out of hand; from mforbes/pytest-flake8

Fixes #8

pytest_flake8.py

@@ -183,7 +183,7 @@ def check_file(path, flake8ignore, maxlength, maxcomplexity,
     app.find_plugins()
     app.register_plugin_options()
     app.parse_configuration_and_cli(args)
-    app.options.ignore = flake8ignore
+    app.options.ignore.extend(flake8ignore)
     app.make_formatter()  # fix this
     app.make_notifier()
     app.make_guide()