Add support and interface for Shortcodes and Plugins

[alfredbaudisch]

Examples:

• https://www.getzola.org/documentation/content/shortcodes/
• https://gohugo.io/content-management/shortcodes/

Syntax proposals:

• Smarty like: https://www.smarty.net/docs/en/language.basic.syntax.tpl

Initial shortcode ideas and proposals:

• Only single-line shortcodes.
• Embed Youtube {% youtube "videoid" %} Embed Tweet {% twitter "tweetid" %}
• List posts from a given taxonomy: {% posts "/books/fantasy/tolkien" %}

This is already possible on a per-application basis. The consuming application can use a post's Post.metadata and do special processing with the metadata.

For example, the demo project has a thumbnail feature.