Add dom testing library

[CarsonMcKinstry]

Reference issue: #83

What changed in this PR:

In order to allow melody-test-utils to simulate most events properly, Kent C Dodd's dom-testing-library has been added and replaces the Event constructor in Wrapper.simulate. This will allow for keyboard and input events to be simulated properly, including being able to test for proper handling of event.target.value.

[pago]

We're missing a newline here. :)

[CarsonMcKinstry]

This should be fixed in the latest commit!

[pago]

dom-testing-library looks pretty amazing. Thank you for setting this up. There's one whitespace missing. Once its there I'll merge it in.

[pago]

I just noticed that dom-testing-library requires node 8 or later. We currently support node 6 or later (that's why the builds fail).
Let me check if we can drop node 6 support already. Otherwise we'd need to make it an optional dependency and throw if not supported. Or skip the test for it if node <= 6.

[CarsonMcKinstry]

Interesting. I guess I got it installed just fine with node 10 and then only did testing with 6+ (didn't re-install).

Let me know what you decide in concerns to node 6 and I can update this PR if need be!