Add additional configuration options.

[nathanhammond]

Closes ember-cli/ember-cli#4586 by providing additional options:

{
  insertContentForTestBody: false,
  useLintTree: false
}

Potentially blocked by ember-cli/ember-cli#4923

Also makes disableContainerStyles require being explicitly set to false. Being undefined means that we should use the default behavior since no settings were changed. This will allow us to remove unnecessary coupling between ember-cli and ember-cli-qunit here.