Allow creating snippets to optimize some builds

[ashmaroli]

• This is a 🙋 feature or enhancement.
• I've added tests.

Problem

Converters do not run on layouts and includes. Therefore, if a user wishes to insert a chunk of Markdown text or code as part of a layout, the workaround is to use {% capture ... %}... followed by rendering the variable through the markdownify filter.
When this routine is part of a layout or include, the probability of rendering the variable numerous times (producing the same markup output) is high.
Such a routine could be optimized if there were a builtin way to have the chunk of Markdown text rendered once and then inserted multiple times.

Solution

Part A -- Jekyll::Snippet

Introduce new entity Jekyll::Snippet that is rendered (like any Page or Document) on demand yet only once per build.

More details in documentation.

Part B -- {% snippet ... %}

Introduce new Liquid Tag {% snippet ... %} to include / insert rendered output of desired snippet.

More details in documentation.