Tab completion in multi-select

### Current result
When using the multi-select option, pressing tab does not select the highlighted result. 

### Expected result
Pressing tab should select the highlighted result. If there is no highlighted result then we should navigate to the next input. Here is a [fiddle](http://jsfiddle.net/_iains/dLezP/embedded/result/) with the desired result. 

Also referenced in issue #1273 

Do people agree this should be the default or would you prefer it to be wrapped in option? Maybe something like `allowsMutliSelectTabCompletion`. 
ditto