Since Flake8 3.0 we've had the ability for plugins to use
`extend_default_ignore` to register codes they want disabled by default.
This, however, was a permanent disabling unfortunately. Our code didn't
have a way of understanding that this new set of `ignore` codes was
actually the 'default' set for that run. Much like the
extended_select_list, we now attach extended_ignore_list to be able to
confidently determine if the ignore we get in the DecisionEngine is
actually the Default Ignore list and what plugins what us to ignore by
default.

Refs https://github.com/PyCQA/pep8-naming/pull/157

Implement `--extend-select` command line argument following what was
done for `--extend-ignore` in !233. This option can be used to
selectively add individual codes without overriding the default list
entirely.

Addresses the remaining item of issue #1061.