Preview links in new posts/comments

...before they are posted of course.

Here's what it looks like when entering a URL for a new post:
![preview-post](https://f.cloud.github.com/assets/64833/8177/9a599ee2-4467-11e2-8239-bb6da5ba9937.png)

And for a new comment:
![preview-comment](https://f.cloud.github.com/assets/64833/8179/9ec458c8-4467-11e2-9f53-f6ce634ac50c.png)