Tagging support

Based on the pull-request of @tandibar #38, I renamed the "add-new-value" to "tagging", and added the management of a function for the tagging attribute.

The tagging can be enabled by adding the attribute "tagging". If set to "true", the search term will be used as the new item. If set to a funciton, the search term will be pass in attribute to the function. For example, I use a function to build a new object from the search term that is like the other objects managed by my ui-select.