Add configurable attributes order

[cepbuch]

closes #8

Added support of manually configurable attributes ordering such as:

[flake8]
class_attributes_order =
    field,
    meta_class,
    nested_class,
    magic_method,
    property_method,
    static_method,
    class_method,
    method,
    private_method

While trying to make it flexible (e.g. not to force users to set all the node types manually), I'm still not sure that my solution is permissible in terms of performance, but looks like it works fine and still no additional complexity was added to "default" and "strict" modes

Any improvement suggestions are accepted with pleasure

[Melevir]

Add a warning from comment above and some unit tests to prove, that new feature works fine and no old behavior is broken.

[Melevir]

Looks like we should fire a warning if both class_attributes_order and use_strict_mode are specified.

[Melevir]

Thanks a lot for the pr! Please provide some additional work on comments above.

[cepbuch]

Fixed!

Also FYI: I had issues with running test locally (the problem is described in the link below). So I guess either attrs==19.1.0 version should be fixed in requirements.txt or pytest updated.

pytest-dev/pytest#3280 (comment)

[Melevir]

Thanks again!