Better handling of embedded images in comments

[cespare]

You can embed images in markdown using this syntax:

![title](http://your/link.png)

This works just fine, except when the image is large it overflows:

We should probably set overflow-x scroll and also it would be good to put a border around them.

[cespare]

Or we could resize the image like I just noticed github does. That's probably better.

[cespare]

And while we're at it, let's make the image an external link (this helps if the image has been resized and you want to view the whole thing).