Leading `<br/>` is not omitted in code blocks

[LucasDachman]

Example from the slack app:

```
some code here
```

produces <div class="slack_code"><br>some code here<br></div> in slack-hawk-down. I'd be happy to look into this if there isn't a quick fix with CSS.

[alechoey]

You'll have to get the opening and closing matches to walk inwards until they find a non-whitespace character.

https://github.com/swiftype/slack-hawk-down/blob/master/src/index.js#L177