This provides a step by step doc for the regular expression, and makes iterating on it much easier

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

```
$ python3.7 -Werror
Python 3.7.0rc1 (default, Jun 16 2018, 03:32:08) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> '# noqa(?:: (?P<codes>([A-Z][0-9]+(?:[,\s]+)?)+))?'
  File "<stdin>", line 1
SyntaxError: invalid escape sequence \s
```

To match our new configuration file format and its allowance for spaces
in the list

Closes #356

* Use set literals instead of set([...])
* Avoid list(sorted(...)) as sorted returns a list
* Replace dict() with dict literal

When we refactored our decision process to fix #239, we broke a subtle
corner case where extensions that are not off-by-default are to be
reported.

This further refactors that logic and adds specific tests around it to
ensure it works as expected and doesn't regress.

Closes #257

This makes the regular expression a bit more complex, and potentially
slower, but it will fix the issue where users had noqa comments with
colons followed by explanations.

Closes #178