Allow custom attributes in images.

This change allows you to pass an additional array parameter to Post::getImage(), to declare HTML attributes you want your image to have, while maintaining backwards compatibility with existing code.

Example: if I want an image to use Bootstrap's `img-responsive` class, I can do it like this:

`{{ $post->getImage('main_image', 'thumbnail', ['class' => 'img-responsive']) }}`
Merge pull request #23 from borfast/patch-1

Allow custom attributes in images.