Add setuptools integration

This proved simpler because I realized something important: Most of the
code that the old integration was using was in fact doing nothing of
value. Since we can't meaningfully allow users to use command-line
options as parameters to `python setup.py flake8`, we just remove the
work that we do to set up those attributes and parse them.

setup.py

@@ -76,8 +76,12 @@ setuptools.setup(
     ],
     install_requires=requires,
     entry_points={
-        'distutils.commands': ['flake8 = flake8.main:Flake8Command'],
-        'console_scripts': ['flake8 = flake8.main.cli:main'],
+        'distutils.commands': [
+            'flake8 = flake8.main.setuptools_command:Flake8'
+        ],
+        'console_scripts': [
+            'flake8 = flake8.main.cli:main'
+        ],
         'flake8.extension': [
             'F = flake8.plugins.pyflakes:FlakesChecker',
             # PEP-0008 checks provied by PyCQA/pycodestyle