Add support for explicit type in meta examples

* This is needed to relate examples to node types for nodes that cannot
  stand alone. Example are :where nodes that do not have syntax outside
  a :case

meta/blockarg.rb

@@ -1,4 +1,4 @@
-Mutant::Meta::Example.add do
+Mutant::Meta::Example.add :blockarg do
   source 'foo { |&bar| }'
 
   singleton_mutations