Two-spaces-then-newline causes line break

```
cabal repl
*Text.Markdown> :set -XOverloadedStrings 
*Text.Markdown> import Text.Blaze.Html.Renderer.Text
*Text.Markdown Text.Blaze.Html.Renderer.Text> let md = renderHtml . markdown def
*Text.Markdown Text.Blaze.Html.Renderer.Text> md "> Hello  \nworld\n\nHow are\nyou?"
"<blockquote><p>Hello<br>world</p></blockquote><p>How are\nyou?</p>"
```
Technically this is slightly off due to not handling CRLF.