Added beforeAddTag callback

This callback can be used for validating format of input/ any other actions to be done before adding tag.

Example: 
inputElement.tagsInput({width: '100%', height: '20px, beforeAddTag: validateEmailAddress})

function validateEmailAddress(tag){
    var emailRegex = /^\w+@[a-zA-Z_]+?\.[a-zA-Z]{2,3}$/;
    if(!tag.val().match(emailRegex)){
      tag.addClass("not_valid");
      return false;
    }else{
      return true;
    }
  }