Handle tight lists

[bert-github]

If no items in a list are separated by a blank line, the <p> tags around text content should be omitted:

- Item 1
- Item 2

should give

<ul>
<li>Item 1</li>
<li>Item 2</li>
</ul>

Unlike

- Item 1

- Item 2

which gives

<ul>
<li><p>Item 1</p>
</li>
<li><p>Item 2</p>
</li>
</ul>

The blank line may occur anywhere in the list and affects all items, even those before it:

- Item 1
- Item 2
- ...
- Item 100

- Item 101