Add a json_c_set_serialization_double_format() function to set the *library-wide* format for how doubles are written to a serialized JSON output.

tests/test_double_serializer.expected

@@ -6,3 +6,6 @@ Test explicit serializer with custom userdata:
 obj.to_string(custom)=test
 Test reset serializer:
 obj.to_string(reset)=0.5
+obj.to_string(default format)=0.52381
+obj.to_string(with global format)=x0.524y
+obj.to_string(with thread format)=T0.52X