Improve UX for customizing theme fonts

[regisphilibert]

Currently the users must rely on tachyon classes added though site.Params.body_classes and site.Params.post_content_classes to customize fonts.

But some have been designed for certain areas of the site which will not inherit from the above. This is a bit confusing.

I think we should simply allow users to define fonts for titles and body and see how to best apply across templates.

cc @Altonss @budparr