If !shouldIncludeStyleguide, exclude the instance initializer

[aaxelb]

I tried using this addon in an engine, and the instance initializer's import Router from '../router'; would error because there is no router there.

I'm not sure the best way to make this fully support engines, but best I could tell the instance initializer is only relevant when the styleguide is enabled. This PR excludes the instance initializer from the app tree if shouldIncludeStyleguide is false, which means it can be used in engines, at least.

[samselikoff]

Thanks so much for this!

Would you be able to update import-files-test.js to assert that the initializer is removed?

[samselikoff]

I prefer not to have early returns, could we change the style to

treeForApp(tree) {
  let appTree = this._super(tree);
  
  if (!this._shouldIncludeStyleguide()) {
    appTree =  new Funnel(appTree, {
      exclude: ['**/instance-initializers/ember-cli-tailwind.js'],
    });
  } 

  return appTree;
}

[aaxelb]

@samselikoff Done 👍

[jrowlingson]

It looks like this change supports item no 2 #17

[samselikoff]

Thank you!

[jrowlingson]

This change is needed for projects with module unification enabled.

[samselikoff]

@jrowlingson it's been released in 0.6.0