Add axe core helper

[notwaldorf]

This PR is bigger than I would have hoped, and I'm sorry. I:

• re-did the whole demo by moving most of the code out of the custom element, and into the main page, and just adding the Redux bits into a custom element
• updated the redux example to always log the state
• added @azakus's axe-report and a demo for it.

The demo for the reporter looks like this:

The demo for the redux bit looks like this:

[keanulee]

I consider this a test helper, not a production app helper, so I don't know how much it belongs in this repo. Welcome other opinions though.

[notwaldorf]

I was thinking that if we make it easily available, and you already added pwa-helpers to your app, then you might be more likely to add some a11y testing to your app (which is part of it being "production" ready imo)

[frankiefu]

What do you think making axe-report.js in its own section (e.g. Test helpers) in the README and in the demo page?

Basic helpers
-----
router.js
network.js
metadata.js
media-query.js

Redux helpers
-----
connect-mixin.js
lazy-reducer-enhancer.js

Test helper
-----
axe-report.js

Btw, love the demo.

[notwaldorf]

I really like the "Test helpers" idea, will add it to the readme and the demo.