descriptionFontDescriptor does not take the font Size into account

### New Issue Checklist

- [x] I updated BulletinBoard to the latest version.
- [x] I read the [Contribution Guidelines](https://github.com/alexaubry/BulletinBoard/blob/master/CONTRIBUTING.md).
- [x] I read the [documentation](https://alexaubry.github.io/BulletinBoard).
- [x] I searched for [existing GitHub issues](https://github.com/alexaubry/BulletinBoard/issues).

### Issue Description
Hello Alex,

I'm trying to set a custom font for the Description Label.
I'm using the property:
`page.appearance.descriptionFontDescriptor`
and I set it like that:
`page.appearance.descriptionFontDescriptor = UIFont(name: "Futura-Medium", size: 12.0).fontDescriptor`

The size remains the default value (which is 20).
However if I set the size, using this attribute:
`page.appearance.descriptionFontSize`
The font size changes, but no as I'm expecting it.
`page.appearance.descriptionFontSize = (UIFont(name: "Futura-Medium", size: 12.0)?.fontDescriptor.pointSize)!`

Am I using the right property to get the font size? `pointSize`

It seems that there is a rate of 1.66666667 or (20/12)
So if I want to font size to be 12, I have to use 20.
If I want it to be 22, it as to be 36.6666667

Is it a know issue? Could you help me resolve it please?

### Environment

- **BulletinBoard Version**: 2.0.1
- **iOS Version**: 11.4
- **Device(s)**: iPhone SE