Choice type parameters raise AssertionError for config files

[xZise]

When using Flake8 2.6.2 together with pycodestyle 2.0.0 and zheller/flake8-quotes 0.7.0 I get the following AssertionError if quotes or inline-quotes are defined as parameter:

$ flake8 setup.py 
Traceback (most recent call last):
  File "/home/xzise/.pyenv/versions/3.5.2/bin/flake8", line 11, in <module>
    sys.exit(main())
  File "/home/xzise/.pyenv/versions/3.5.2/lib/python3.5/site-packages/flake8/main.py", line 28, in main
    flake8_style = get_style_guide(parse_argv=True)
  File "/home/xzise/.pyenv/versions/3.5.2/lib/python3.5/site-packages/flake8/engine.py", line 234, in get_style_guide
    styleguide = StyleGuide(**kwargs)
  File "/home/xzise/.pyenv/versions/3.5.2/lib/python3.5/site-packages/flake8/engine.py", line 154, in __init__
    self._styleguide = kwargs.pop('styleguide', NoQAStyleGuide(**kwargs))
  File "/home/xzise/.pyenv/versions/3.5.2/lib/python3.5/site-packages/pycodestyle.py", line 1833, in __init__
    arglist, parse_argv, config_file, parser)
  File "/home/xzise/.pyenv/versions/3.5.2/lib/python3.5/site-packages/pycodestyle.py", line 2120, in process_options
    options = read_config(options, args, arglist, parser)
  File "/home/xzise/.pyenv/versions/3.5.2/lib/python3.5/site-packages/pycodestyle.py", line 2076, in read_config
    assert opt_type in ('store_true', 'store_false')
AssertionError

This is because the option's type is choice and that isn't actually handled. It should be handled the same as string as it is handled the same by optparse.

So instead of elif opt_type == 'string': it could just do elif opt_type in ('string', 'choice'):. This will also verify that the choice is actually correct so no additional changes would be required.