Make document classes configurable

[ronaldtse]

Different organizations use slightly different document models.

We want to allow them to configure their own document schema via a configuration file. This should not require authoring a gem but a remote document schema file (determine which classes to adopt and how they behave differently).

[hmdne]

The idea I have would be a DSL to generate document fragments. Like, let's say

builder.build {
  paragraph {
    children {
      text_node "abc"
      anchor "http://example.com", "hello"
      text_node "abc"
    }
  }
}

This would dispatch individual calls to classes like Coradoc::Element::Paragraph. But if we override the builder, it could give different classes for those calls. So, such a schema file, could be a DSL to configure said builder, or some other kind of config.