feat: adds search placeholder configuration (#613)

* implements search placeholder configuration

* changes implementation approach to use search_placeholder_custom

Thanks to @pdmosses for the suggestion!

* strips whitespace and HTML for both placeholder and aria-label

_includes/search_placeholder_custom.html

+Search {{site.title}}

_layouts/default.html

@@ -92,9 +92,12 @@ layout: table_wrappers
   <div class="main" id="top">
     <div id="main-header" class="main-header">
       {% if site.search_enabled != false %}
+
+        {% capture search_placeholder %}{% include search_placeholder_custom.html %}{% endcapture %}
+
         <div class="search">
           <div class="search-input-wrap">
-            <input type="text" id="search-input" class="search-input" tabindex="0" placeholder="Search {{ site.title }}" aria-label="Search {{ site.title }}" autocomplete="off">
+            <input type="text" id="search-input" class="search-input" tabindex="0" placeholder="{{ search_placeholder | strip_html | strip }}" aria-label="{{ search_placeholder | strip_html| strip }}" autocomplete="off">
             <label for="search-input" class="search-label"><svg viewBox="0 0 24 24" class="search-icon"><use xlink:href="#svg-search"></use></svg></label>
           </div>
           <div id="search-results" class="search-results"></div>