Add `pre` and `post` menu params

so that icons can be added via external scripts (ex. fontawesome)

fixes: #420

ref: https://gohugo.io/content-management/menus/#params

layouts/partials/header.html

@@ -148,7 +148,9 @@
                 <a href="{{ .URL | absLangURL }}" title="{{ .Title | default .Name }} {{- cond $is_search (" (Alt + /)" | safeHTMLAttr) ("" | safeHTMLAttr ) }}"
                 {{- cond $is_search (" accesskey=/" | safeHTMLAttr) ("" | safeHTMLAttr ) }}>
                     <span {{- if eq $menu_item_url $page_url }} class="active" {{- end }}>
+                        {{- .Pre }}
                         {{- .Name -}}
+                        {{ .Post -}}
                     </span>
                 </a>
             </li>