Add support for flake8

[adius]

https://pypi.python.org/pypi/flake8

As seen in https://github.com/thread/routemaster/pull/41#issuecomment-367622211

Configuration is stored in setup.cfg

[danpalmer]

Configuration can be stored in many different files. setup.cfg is probably the most common for flake8, but there are other valid places, and some flake8 plugins may also use config from other places. For example, flake8-isort calls into isort which many people configure through an isort.cfg plugin (even though it can be configured through setup.cfg).

Worth noting that most uses of flake8 I've seen use a custom set of plugins, often pinned to versions, so any automated system would need the ability to pickup that list of dependencies and install them. This has been the main blocker to hosted Python linting solutions in the past, and one of the reasons we just fail our build when there are linter violations.

[adius]

Wow, sounds complicated 😅. Thanks for the heads up. Maybe we can support a subset of the features for the beginning and ignore the rest. (The big advantage of Feram is no setup time, and much faster than CI. So if Feram already fails before your CI you can can just cancel the build and fix the bug before retrying. Saves a lot of time. So even just a subset is helpful for the beginning)