Introduce some templating for include_code in liquid_tags

Built on top of #321 (fixing #312). Most of the include_code snippets are hardcoded, and not suitable for
theming or translation. Addition of a `classes` parameter per-file and
the following config variables help resolve the situation:

 * CODE_OPENTAG: Opening template for include_code subplugin
 * CODE_CLOSETAG: Closing template for include_code subplugin
 * CODE_DOWNLOADSTRING: The "download" helper string for include_code subplugin