New containerProps prop

[gpbl]

This new prop remove the previous behavior of props being spread to the container's <div /> element by default. Instead, developers should use containerProps:

<DayPicker 
-    data-my-thing="foo" 
+    containerProps={ 'data-my-thing': 'foo' }
/>

This will avoid to loop over the prop types to remove the invalid ones, improving rendering performance as suggested here.

[coveralls]

Coverage remained the same at 100.0% when pulling cd980ab on container-props into ecc30be on master.