Most of the problems with our logic were due to not having the same
logic as our exclude parameter. This refactors that out into a separate
function so we can confidently achieve that.

This adds support for rules that ignore violation codes on a per-file
basis. This takes a similar functional approach to
https://github.com/snoack/flake8-per-file-ignores which allows for glob
patterns like the `--exclude` option.

Closes #156

Instead of just using Flake8 and pylint to keep Flake8 clean, let's also
use black to make it less manual for clean-up.

By slightly modifying our utility function to parse comma separated
lists we can parse local plugins similar to other configuration options.

Closes #329

When we parse out our comma separated lists, we should ignore empty
strings to avoid them breaking users' expectations.

Closes #330

When we read in a file on Python 3, we use the tokenize module to detect
the coding pragma at the top of the file. We then use that to decode the
rest of the file. However, when we were receiving stdin, we would not do
that.

This updates ``stdin_get_value`` in a backwards compatible way to check
for that coding pragma and then fall back to UTF-8 if necessary.

Closes #306

Due to https://bugs.python.org/issue27649, we cannot continue to
expect multiprocessing to work as we expect and document it on Windows.
As such, we are going to revert back to our previous behaviour of
disabling it across all versions of Python on Windows to provide the
default expected behaviour of Flake8 on that Operating System.

As FichteFoll pointed out on IRC, this line could include Python 3s
less than 3.2 as well.