add an option to let user use tab as enter

By default, the tab navigates the focus to the next input, which is not many of us want. 
If we want to use the Tab key as "confirm a tag input" like the Enter key does, we can turn on the preventNavigationOnTab to do so in this pull-request. The Tab key will still navigate if the input is empty. This behavior is commonly used in modern websites and software like facebook and chrome.