theme: support conditional content

[McShelby]

There are use cases where the content of a site should be filtered on built to remove certain content a user should not see.

For example, a product documentation contains every feature in its Markdown files, but for a certain customer only a few of them are installed. The generated project should only contain documentation for the available features.

Implementationwise this will be implemented in a new shortcode that is flagged with a feature array in its parameter, similar to tags in frontmatter. If at least one of this feature flags is activated in the params of the config.yaml, the content will be put in the resulting page.

See:
https://discourse.gohugo.io/t/ignore-content-when-publishing/13008/4
https://discourse.gohugo.io/t/render-some-pages-only-in-non-production/44306/5
https://discourse.gohugo.io/t/cli-filter-content-for-different-target-audiences/15439
https://discourse.gohugo.io/t/omitting-content/26437