Allow html attributes

[SergeAstapov]

The SimpleDOM document that gets created during fastboot rendering has a html tag, and it's possible to set attributes on it. For example, assuming getDOM from ember-wormhole:

Component.extend({
  init() {
    this._super(..arguments);
    getDOM(this).documentElement.setAttribute('data-foo', 'hello')
  }
})

But this doesn't actually cause any output in fastboot, because the user's own tag from index.html is always used instead.

This PR allows attributes set on the SimpleDOM html to be copied into the output.

Credit to @ef4 for inspirational PR #171.

Closes ember-fastboot/ember-cli-fastboot#331

[SergeAstapov]

Hi @kratiahuja! Do you mind to take a look once get a chance? Do you feel this is right way to approach the problem?

[bobisjan]

This would be a great addition for using lang attribute on HTML element 👍.

[rwjblue]

Sorry for the delay here! Merged and released as 1.2.0...