Allow Addons to Add Icons to the Build

[jrjohnson]

Instead of relying on the app to discover and include needed icons, this
allows each addon to register their needs in config/icons.js and they will be
picked up by the app and added to the final build.

Hoping for feedback on the file location and the format before I write any docs. Addons would supply icons as a module:

module.exports = function() {
  return {
    'free-solid-svg-icons': 'all',
    'free-brands-svg-icons': [
      'black-tie'
    ],
  };
};

I've elected to wrap it in a function in case we want to provide arguments in the future. Other addons like ember-intl consolidate configuration like this and I think it is a good pattern here.

Another option I considered would be to read any icon configuration in the addon config/environment.js and bundle it into the app, but ember-cli currently does a shallow merge of those options into the app which wouldn't include combining the icons options together. I feel like having our own configuration file is actually a bit less surprising than having a single option in the addons environment.js file which is magically merged into the app configuration.

Steps after this is complete would include:

1. Deprecating the icons option in the environment.js of apps and addons
2. Auto detecting static icon names in templates so addons don't need to list everything manually or creating some kind of test integration like ember-cli-deprecation-workflow does that would find icon names and make it easy to create the icons.js file (or maybe both!)

[robmadole]

This is fantastic. Nice work. The file format and the way it operates looks great.