Add W3C KeyboardEvent.key values support.

[leocavalcante]

As discussed at #7

KeyboardEvent.keyCode is deprecated, also MDN recommends to avoid it for new projects in favor of KeyboardEvent.key.

This PR is a result of following this recommendations without dropping the support for keyCode since not every browser has implemented the new standard yet.

The usage of keyValue instead of keyId as discussed at #7 is to match an ubiquitous terminology with current standards.

Now, despite letter case, the arrow keys values are the only difference I notice from keycodes naming to actual key values so keyNameVals might seams dummy, but it allows to map any another differences.

Hope it helps!
Cheers

[leocavalcante]

Sorry about the flow mess. Just setup a Linux VM to properly test the project.

[ayrton]

Awesome work Leo, this is epic stuff 🙌

[ayrton]

I would name this keyValueFromCode and move it to lib/utils.js

[leocavalcante]

@ayrton, thanks and I do agree with all of your appointments, so here are them.
Glad to help! Learned a lot playing with this, I appreciate, loved this Flow thing, static analyses FTW!

[ayrton]

Glad you enjoyed it and you were able to learn something from this.

[ayrton]

Merged this in, will try to release a new version in the next couple days, thank you for your contribution!