Allow text between header and module content

[DocSkellington]

For now, the output is only the section's title and the content generated from the JSON document. It should be possible to add an user-defined text between the two. Moreover, it should be an instance of cvbuilder.modules.Description. For instance,

module = ModuleName(section="Example", description=Description("This is an example of a text between the section's title and the content."))

produces a section called Example, followed by the context-appropriate representation of the description. Finally, the module's main content is produced.