Extend metadata coverage in views

Rationale / Begründung

RDMO offers a <metadata> tag to add metadata to views. This sounds good but has some severe elimitations:

1. It works only with "fixed" content, not with Django variables.
2. It covers only the pandoc title block. i.e. the three fields title, authors and date.
3. These three fields are always reproduced as free text, even when they are not required.

Affected

This improvement would favour managers and end users in the following scenario:

Funding agencies often require a DMP formatting according to a very precise template.
The template is impossible to reproduce with Django, because special formatting is lost during the Pandoc conversion.
Some elements such as the titlepage can be successfully input into the Pandoc script with the help of a "reference doc".
Variable information must then be provided in another way. For example using form fields in Microsoft Word, which can be associated to metadata fields.

Minimal functionality

It should be possible to add a small set of metadata to a HTML/Django document, so that it is propagated by Pandoc to another output format. The additional fields which might be propagated lossless are: subject, description, keywords and category. (The list is so short because of the very small conformity of Microsoft's own scheme with the universal scheme Dublin Core.)

A syntax should also be defined which allows inclusion of Django-like variables in the metadata fields.