Add syntax highlighting for code blocks

Fixes #7

To use code blocks, fence the code just like in github markdown:

```js
var x = 2;
++x;
function mult(x, y)
{
   return x * y;
}
```

I initially was going to use [marked](https://github.com/chjj/marked), but I
don't want all the functionality
marked offers, just some of it. There's no way (as far as I can see) to include
just a few of the markdown commands.

So I added the biggy: code blocks. Bolding, colors, etc will probably be added
later. I'm debating whether I should use bbc(?) (forum-style) markup or
other.. But right now that stuff isn't too desireable.