Allow users to preview the Markdown they've written before publishing.
[Also tracked on Trello](https://trello.com/c/wsKTBuRO/65-real-time-markdown-preview).
**mrspirateowl** mentioned this [on Twitter](https://twitter.com/mrspirateowl/status/988689857533931520):
> you know what else I'm missing? previews
> esp. when I'm adding a photo
Add a dropdown menu on the publish button and a **Preview** button that replaces the editor with a full preview of the post. Have some way to immediately switch back to editing.