Allow updater function in bootstrap-typeahead.js (updates the value of the input field) to be overridden

Created by: iamnirav

This update separates out the code that updates the input field once a user selects a typeahead item. It also adds in an option for a developer to override this function. This means you can specify something like

var updater = function (item) {
  var tokens = this.query.split(' ');

  // Replace last (incomplete) token with completed selection
  tokens.splice(-1, 1, item + ' ');

  return tokens.join(' ');
}

to have a typeahead where you can string together a chain of autocompleted tokens.