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

In working on flake8-json, I noticed that setting newline was only
helpful for writing to an ouput-file but not to standard-out. It makes
sense for this setting to apply to both branches otherwise plugins need
to override the method to get the behaviour they expect.

Move all Violation related methods from the StyleGuide to our Violation
class.

This adds two new methods to the BaseFormatter class:

- beginning
- finished

These will indicate when Flake8 begins and finishes processing a file.

Closes #251

When users specify any number of -q's on the command-line, we should not
show the source even if they have otherwwise configured Flake8 to do so.

Closes #245

Add --tee option to split report output stream.

The `--tee` option allows the linter report to be written to stdout, even
though it is being redirected to a file with the` --output-file` option.
This is useful if I want to store the report in a separate file for later
analysis but also be able to print the output on screen (e.g when running
in a CI environment).

See merge request !90