html: Fix headline priority export

when #+OPTIONS: pri:t is set (org-export-with-priority) Org mode exports the
priority as {[A], [B], [C]}, not {A, B, C} - we should do the same

org/html.go

@@ -206,7 +206,7 @@ func (w *HTMLWriter) writeHeadline(h Headline) {
 		w.WriteString(fmt.Sprintf(`<span class="todo">%s</span>`, h.Status) + "\n")
 	}
 	if h.Priority != "" {
-		w.WriteString(fmt.Sprintf(`<span class="priority">%s</span>`, h.Priority) + "\n")
+		w.WriteString(fmt.Sprintf(`<span class="priority">[%s]</span>`, h.Priority) + "\n")
 	}
 
 	w.WriteString(title)

org/testdata/headlines.html

@@ -22,7 +22,7 @@ not just where they are actually meant to be - even here &gt; <code class="stati
 </ul>
 <h1>
 <span class="todo">TODO</span>
-<span class="priority">B</span>
+<span class="priority">[B]</span>
 Headline with todo status &amp; priority
 </h1>
 <h1>
@@ -33,7 +33,7 @@ Headline with TODO status
 the <strong>content</strong>
 </p>
 <h1>
-<span class="priority">A</span>
+<span class="priority">[A]</span>
 Headline with tags &amp; priority&#xa0;&#xa0;&#xa0;<span class="tags"><span>foo</span>&#xa0;<span>bar</span></span>
 </h1>
 <p>