Warn of inconsistent use of P tags within an LI

[jidanni]

See that line without a <p> tag hiding in the middle there?:

   <li>
     <h3 id="do_not_blame">別怪地圖</h3>

     <p>好， 若不要製圖單位大膽的把【中正路】寫在馬路上， 那麼就只好寫在房子上。 每房子標全址，
     圖就壅塞。</p>
     This line has no "p" tag!
     <p>大多數Ｘ路Ｙ號的房子前邊就是Ｘ路。 除非要很多人工介入， 判別。

Well, sure, we all make lists with no P tags all the time.

<UL><LI><LI><LI></UL>

However when we do use P tags within an LI, we expect we use them consistently.

So tidy should warn us when we don't!