Fixes #260

* Use keyword `warn` in eslint config

We can use the keyword `warn` for rule configuration instead of a number.

* Fix comment which where WARNING was replaced

* Remove extra apostrophe

This project is a really good use case to apply some accessibility
rules to the lint config. For beginners getting started with React, we
can also teach/enforce basic accessibility rules in the web at author
time.

For now, I’ve just applied to the rules that are listed in
`eslint-config-airbnb` because these are most real-world tested, and we
can continue to add.

Happy to help if there are any questions/issues/concerns about the
plugin!

Ref #156

Not sure if we can simply delete the line and that'll disable it, so I
went with setting it to 0. Let me know!

* Make errors and warnings nicer

* Fix license stripping to not be eager

* Minor tweaks

* Use a custom eslint rule list

* Sort rules

* Relax some rules