Add displayKey as valid option

as considered here #127

Author: holtkamp

doc/jquery_typeahead.md

@@ -155,7 +155,7 @@ Datasets can be configured using the following options.
 
 * `limit` – The max number of suggestions to be displayed. Defaults to `5`.
 
-* `display` – For a given suggestion, determines the string representation 
+* `display` | `displayKey` – For a given suggestion, determines the string representation 
   of it. This will be used when setting the value of the input control after a 
   suggestion is selected. Can be either a key string or a function that 
   transforms a suggestion object into a string. Defaults to stringifying the