Class for parsed templates

[davidchall]

If a template will be rendered multiple times, it makes sense to parse the template once and store the intermediate object.

• parse(.x, .config) returns an S3 object with class "jinjar_template".
• render.jinjar_template(.x, ...) renders the template like usual.
• The class stores:
  • a cpp11::external_pointer to the inja::Template object (i.e. after parsing).
  • The jinjar_config object (needed again for rendering).