Fix hr rendering

Now earmark renders hr with escaped `\n` at the end and produce extra symbols [in html](http://artemeff.com/exredis/README.html):

```elixir
Earmark.to_html("---")
# => "<hr/ class=\"thin\">\\n"
```

I'm fix it:

```elixir
Earmark.to_html("---")
# => "<hr/ class=\"thin\">\n"
```