remove update_policy from YAML frontmatter
remove enum-lang-posts.html
post-list: use include with param instead of includex
remove includex plugin

Official include plugin now support parameters.
remove includex plugin, update_plicy & enum-lang-posts.html template
remove unused include files
add content_ext plugin & remove post_to_liquid_raw plugin
remove json_filter plugin (official jsonify filter is released)