Fixes URLs in strings

Hi Lea,

I had a lot of `url`s in strings with no highlighting (like you mention [here]), and the *auto-linker* failing.

### This solution

This *PR* makes it work wo/ any side effects (AFAIK).

### Another solution

I also tried to prioritize the `string` regexp over the `comment` one.
This also resolves the problem + the first failure you mention (comment-like substrings).

But there is another side effect: comments with quotes inside, now have strings highlighted (like regexp).

Let me know if you prefer the other solution.
And btw, thanks for your work :)

[here]: http://prismjs.com/examples.html#failures