HN app: Replace p tags in comment body by newline characters

[rauanmayemir]

As marked in https://github.com/briskml/brisk/blob/master/examples/hacker-news/app/Story.re#L82, we'll need to transform incoming html.

<p> tags are not rendered with proper margin/padding by Cocoa in NSTextView by default, and instead of manually changing document attributes on ObjC bindings side, simply pre-processing html text and replacing paragraphs by newline characters seems more preferable.

We already use markup.ml, so it shouldn't be hard to do the adjustments. The relevant part to look at is http://aantron.github.io/markup.ml/#VALtree