Don't wrap simple titles in <p>

Before this commit, if an org document was titled "Title here", the
first line of HTML output would be as follows:

<h1 class="title"><p>Title here\n</p></h1>

This commit changes the HTML writer to instead output the following:

<h1 class="title">Title Here</h1>

I conversatively modified the code, so there might be more cases where
elements should be omitted from the title.

org/testdata/misc.html

@@ -1,5 +1,4 @@
-<h1 class="title"><p>Misc title <b>with an inline html export</b></p>
-</h1>
+<h1 class="title">Misc title <b>with an inline html export</b></h1>
 <nav>
 <ul>
 <li><a href="#headline-1">issues from goorgeous (free test cases, yay!)</a>