I'm not sure where this code went or when, but it disappeared. Let's
add it back.

Related #180

Since the "git config" command adds a newline to the end of its output, the extraneous whitespace needs to be stripped out for proper parsing.

Fixes #170

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

We need to initialize part of the Application so we can set options
passed by the user, but we also want to delay making things, e.g.,

- Formatter
- Style Guide
- etc.

Until we have the options solidified so we don't have to do annoying
things.

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

This does not handle setting custom options via the parameters to
get_style_guide.

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