Update ng-tags-input.js

Replaced the manual search filter you were using with an angular filter. This will, later, allow an option for users to provide their own filter function. I have not done that yet - as I figured you'd want to see what/if any issues this change raises. As is atm, the filter function I included lowercase as you did but, using angular.lowercase it loses the issues with regular js lowercase comparisons for other locales. aka - this lowercase works cross locale :)

If you're fine with this concept, let me know if you want me to go the next step and add the user filter injection option :)