code: function mixin(target, source, force, deepStringMixin) {
          if (source) {
              eachProp(source, function (value, prop) {
                  if (force || !hasProp(target, prop)) {
                      if (deepStringMixin && typeof value !== 'string') {
                          if (!target[prop]) {
                              target[prop] = {};
                          }
                          mixin(target[prop], value, force, deepStringMixin);
                      } else {
                          target[prop] = value;
                      }
                  }
              });
          }
          return target;
      }
docs: Simple function to mix in properties from source into target,
  but only if target does not already have a property of the same name.