Draft: Show complete article with rdrview

NOTE: This should not be merged. It's just a basis for discussion.

Many feeds do not contain the complete article text but only a teaser (e.g. https://www.heise.de/rss/heise.rdf). It is useful to fetch the complete article text such that you don't have to open the feed in the browser.

This proof of concept uses rdrview to achieve this.

Questions:

• Shall such a feature be added to Alligator?
• Shall it be part of Syndication?
• How should it be implemented?

See also this pending rdrview issue to provide a library: https://github.com/eafer/rdrview/issues/19.

Alternative solutions:

• Use mozilla/readability. This could be done in C++ with QScriptEngine or in qml.