don't add GFM line breaks inside non-paragraph content

[chrisvasz]

When rendering a document with GFM line breaks turned on, the following

# hello

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

# world

renders as

<h1 id="hello">hello</h1>
<ul><br>  <li>1</li><br>  <li>2</li><br></ul>

<h1 id="world">world</h1>

According to this line from the GFM help page (emphasis added):

GFM treats newlines in paragraph-like content as real line breaks, which is probably what you intended.

... the <br> tags should only be added in "paragraph-like content".

[Feder1co5oave]

+1, breaks are quite tricky

[joshbruce]

#985