Add support for local (in-repo, non-setuptools) plugins.

Closes #357

tests/integration/test_plugins.py

@@ -0,0 +1,58 @@
+"""Integration tests for plugin loading."""
+from flake8.main import application
+
+
+LOCAL_PLUGIN_CONFIG = 'tests/fixtures/config_files/local-plugin.ini'
+
+
+class ExtensionTestPlugin(object):
+    """Extension test plugin."""
+
+    name = 'ExtensionTestPlugin'
+    version = '1.0.0'
+
+    def __init__(self, tree):
+        """Construct an instance of test plugin."""
+        pass
+
+    def run(self):
+        """Do nothing."""
+        pass
+
+    @classmethod
+    def add_options(cls, parser):
+        """Register options."""
+        parser.add_option('--anopt')
+
+
+class ReportTestPlugin(object):
+    """Report test plugin."""
+
+    name = 'ReportTestPlugin'
+    version = '1.0.0'
+
+    def __init__(self, tree):
+        """Construct an instance of test plugin."""
+        pass
+
+    def run(self):
+        """Do nothing."""
+        pass
+
+
+def test_enable_local_plugin_from_config():
+    """App can load a local plugin from config file."""
+    app = application.Application()
+    app.initialize(['flake8', '--config', LOCAL_PLUGIN_CONFIG])
+
+    assert app.check_plugins['XE'].plugin is ExtensionTestPlugin
+    assert app.formatting_plugins['XR'].plugin is ReportTestPlugin
+
+
+def test_local_plugin_can_add_option():
+    """A local plugin can add a CLI option."""
+    app = application.Application()
+    app.initialize(
+        ['flake8', '--config', LOCAL_PLUGIN_CONFIG, '--anopt', 'foo'])
+
+    assert app.options.anopt == 'foo'