Extract hint tags from <textarea>

This is similar to <input> and also often uses a placeholder or name to
identify itself.

qutebrowser/browser/hints.py

@@ -1053,6 +1053,7 @@ class WordHinter:
             "img": ["alt", "title", "src"],
             "a": ["title", "href", "text"],
             "input": ["name", "placeholder"],
+            "textarea": ["name", "placeholder"],
             "button": ["text"]
         })