Support for code listing caption

[hhamon]

It would be useful to be able to specify a caption for a code listing.

I suggest to use a syntax similar like this one:

[code php "Listing 2-3: class skeleton with constructor"]
<?php
class Foo
{
    private $bar;

    public function __construct($bar)
    {
        $this->bar = $bar;
    }
}