Improve merging of code & doc data.

No more assume everything's coming from doc - also look at the
code data and merge stuff from there that didn't came from doc.

Make use of this to implement @evented and @accessor as BooleanTag
subclasses.

lib/jsduck/builtins/accessor.rb

-require "jsduck/builtins/tag"
+require "jsduck/builtins/boolean_tag"
 
 module JsDuck::Builtins
-  class Accessor < Tag
+  class Accessor < BooleanTag
     def initialize
-      @pattern = "accessor"
-    end
-
-    # @accessor
-    def parse(p)
-      p.add_tag(:accessor)
+      @key = :accessor
+      super
     end
   end
 end

lib/jsduck/builtins/evented.rb

-require "jsduck/builtins/tag"
+require "jsduck/builtins/boolean_tag"
 
 module JsDuck::Builtins
-  class Evented < Tag
+  class Evented < BooleanTag
     def initialize
-      @pattern = "evented"
-    end
-
-    # @evented
-    def parse(p)
-      p.add_tag(:evented)
+      @key = :evented
+      super
     end
   end
 end

lib/jsduck/doc_ast.rb

@@ -90,8 +90,6 @@ module JsDuck
         :doc => detect_doc(docs),
         :default => detect_default(:cfg, doc_map),
         :properties => detect_subproperties(:cfg, docs),
-        :accessor => !!doc_map[:accessor],
-        :evented => !!doc_map[:evented],
       }, doc_map)
     end
 

lib/jsduck/merger.rb

@@ -81,6 +81,10 @@ module JsDuck
       docs.each_pair do |key, value|
         h[key] = docs[key] || code[key] || defaults[key]
       end
+      # Add items only detected in code.
+      code.each_pair do |key, value|
+        h[key] = value unless h.has_key?(key)
+      end
 
       h[:name] = merge_name(docs, code)
       h[:id] = JsDuck::Class.member_id(h)