Multiple search boxes support - fixes #49 (#52)

Changes how the searchbox template gets the value of searchbox inputs to permit multiple search boxes on a page.

templates/partials/simplesearch_searchbox.html.twig

@@ -5,16 +5,15 @@
     {% endif %}
     <script>
     jQuery(document).ready(function($){
-        var input = $('[data-search-input]');
-
-        input.on('keypress', function(event) {
+        $('.search-input').on('keypress', function(event) {
+            var input = $(event.currentTarget);
             if (event.which == 13 && input.val().length >= {{ config.get('plugins.simplesearch.min_query_length', 3) }}) {
                 event.preventDefault();
                 window.location.href = input.data('search-input') + '{{ config.system.param_sep }}' + input.val();
             }
         });
-
         $('.search-submit').on('click', function(event) {
+            var input = $(event.currentTarget).prev('.search-input');
             event.preventDefault();
             window.location.href = input.data('search-input') + '{{ config.system.param_sep }}' + input.val();
         });