Fix error message
Include templating option for include_code
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

The default values leave the default "theme" as is.
Merge branch 'feat_includecode_utf8' into local
Update documentation
Merge branch 'feat_includecode_templating' into local

Conflicts:
	liquid_tags/Readme.md
	liquid_tags/include_code.py
	liquid_tags/mdx_liquid_tags.py