Add default value option to combineReducers

[zhigang1992]

Adding a defaultValue option to combineReducer.
It's pretty useful when you want to provide a initial state for it. :)

[markerikson]

Thanks for the submission, but similar changes have been proposed and rejected in the past. See #1904 (comment) for a recent discussion, as well as Dan's explanation of how to initialize data in reducers at http://stackoverflow.com/questions/33749759/read-stores-initial-state-in-redux-reducer/33791942 .