Compile and cache a template from its source string.
Cache key (rule index or named template).
Handlebars template source.
The compiled template.
Get a previously compiled template by key.
The cache key.
The compiled template, or undefined.
Render a compiled template against a context.
The cache key of the compiled template.
The data context for rendering.
The rendered string, or null if the template was not found.
The template engine: holds compiled templates and renders them against context.