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

This avoid overwriting portions of our log output when using Flake8
in verbose mode.

Closes #193

We accidentally changed column numbering from one-indexed to
zero-indexed.

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).

Fundamentally on Flake8 2.x using -q altered the format of the errors
(and the behaviour a little) so it makes the most sense to implement
this logic with formatters rather than messy logic spread throughout
the project.

The FilenameOnly formatter will keep track of filenames already reported
and only print the name once while Nothing will print nothing.

Closes #180

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

Related #180

Some plugins (e.g., flake8-future-import) report errors for empty
files. Those plugins default to reporting the line number as 1 which
caused earlier versions of Flake8 3.0 beta to crash on an IndexError

Closes #157

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