If a user specified `--max-complexity` on the command-line, they
would be told that it did not exist. The same would be true of any
option provided by a plugin. This is because we parse the command-line
arguments twice in Flake8 -- the first time to specify the verbosity
and destination for logging, the second time to actually execute Flake8.
Since plugin options are not registered to start with the first time,
they are not valid options. So when we first parse the options, we should
only attempt to parse the ones which we know about.

Closes #168

When flake8's config is in setup.cfg, setuptools attempts to set those
options on the command instance. If they don't exist, it fails early
complaining that a specific option does not exist.

This adds this back and does it better than the Flake8 2.x version.

Closes #163

The intended behaviour already existed.

This reverts commit db9d4ad8b4cc2aab1ef8349dc969faa8f589e9cc.

On Flake8 2.x we added the information about the implementation,
version, and operating system to the --version output to make helping
users easier. In short they can pretty simply just give us the output
from

    flake8 --version

And we can get a lot of the information that we need.

Previously the --select was only ever populated to E,F,W,C and so
plugins would not be reported when not off-by-default. This adds a
tiny shim so that we enable plugins that are not off-by-default and
:x
:x

Previously Flake8 parsed both

    max-line-length = 110

And

    max_line_length = 110

From the config file without issue. When we updated our logic, I forgot
to test for that and we lost that behaviour temporarily.

Closes #152

If somebody explicitly has a `.flake8` file, presumably they intend to
put flake8 configuration in it, so prefer it to the generic `setup.cfg`
and `tox.ini` from pycodestyle.

This is an emerging best practice and there is little reason to not
follow it