[qa] Introduce QA checks

[nemesifier]

In the other open source python packages maintained by OpenWISP, we have a pretty consistent set of QA checks which we could reuse here:

• black formatter
• isort
• flake8
• commit message check

Here's some examples of how we use it:

• https://github.com/openwisp/netjsonconfig/blob/master/run-qa-checks
• https://github.com/openwisp/netjsonconfig/blob/master/.travis.yml#L20-L21

I think we should introduce these. I realized after seeing #229.

[devkapilbansal]

I want to work on this issue.

[nemesifier]

I want to work on this issue.

Sure, go ahead 👍