FEATURE: support registration of custom html by plugins

lib/discourse_plugin_registry.rb

@@ -12,6 +12,7 @@ class DiscoursePluginRegistry
     attr_accessor :desktop_stylesheets
     attr_accessor :sass_variables
     attr_accessor :handlebars
+    attr_accessor :custom_html
 
     # Default accessor values
     def javascripts

lib/plugin/instance.rb

@@ -108,6 +108,11 @@ class Plugin::Instance
     @javascripts << js
   end
 
+  def register_custom_html(hash)
+    DiscoursePluginRegistry.custom_html ||= {}
+    DiscoursePluginRegistry.custom_html.merge!(hash)
+  end
+
   def register_asset(file, opts=nil)
     full_path = File.dirname(path) << "/assets/" << file
     assets << [full_path, opts]