More accurate intro generation.

Sometimes post paragraphs are wrapped by another tag, such as
<blockquote>, now it will correctly add their contents to intro and
close all tags after limit is reached.

Post example:

```markdown
First paragraph.

> Quote begins, first paragraph
>
> Second par.
>
> Third one.
```

If we have maxparagraphs=3, before it would produce HTML like that:

```html
<p>First paragraph.</p>
<blockquote><p></p><p></p>
```