Make guidemaker service more flexible

[BlueCutOfficial]

Currently, guidemaker service defines fixed properties like title or logo out of the environment config. It restricts the possibility of customizations when using guidemaker in a custom guidemaker's template.

It would be nice to make guidemaker service act as a simple proxy to access the config, so we can do for instance:

my-app/config/environement.js

module.exports = function (environment) {
  let ENV = {
    guidemaker: {
      title: 'My app built with guidemaker',
      customProp: 'a property I need to customize my guidemaker template'
    }
  }
}

<!-- my-guidemaker-template/templates/application.hbs -->

{{this.guidemaker.customProp}}

We have a first use case with guidemaker-ember-locale-template-ember-fr-guides-source pair: we use a mascots property to prepare the use of locale mascots, and we'll probably use some new properties to customize the footer and maybe other layout elements.