Make guidemaker service more flexible

Currently, [guidemaker service](https://github.com/empress/guidemaker/blob/main/addon/services/guidemaker.js) 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:
```js
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'
}
}
}
```

```hbs

{{this.guidemaker.customProp}}
```

We have a first use case with [guidemaker-ember-locale-template](https://github.com/DazzlingFugu/guidemaker-ember-locale-template)-[ember-fr-guides-source](https://github.com/DazzlingFugu/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.