Make image responsive: optional attribute added to not include height and width attributes in HTML markup

I'm using your plugin in a responsive template. To make images adaptive, I needed the `img` markup free from `height` and `width` attributes.

So I added an optional attribute (`hw_markup`) which controls the image HTML output.

`hw_markup` accepts `0` or `1` values.

Here is how it works:

```
// Outputs default markup:
echo dynamic_image_resize( array(
	'src' => array_shift( wp_get_attachment_image_src(
			get_post_thumbnail_id( get_the_ID() ),
			"Full"
		) ),
	'width'   => 60,
	'height'  => 101,
	'classes' => 'thumb',
	'hw_markup' => 1
 ) );

// Outputs markup without width/height attrs:
echo dynamic_image_resize( array(
	'src' => array_shift( wp_get_attachment_image_src(
			get_post_thumbnail_id( get_the_ID() ),
			"Full"
		) ),
	'width'   => 60,
	'height'  => 101,
	'classes' => 'thumb',
	'hw_markup' => 0
 ) );
 
// hw_markup attribute omitted: outputs image with standard markup
echo dynamic_image_resize( array(
	'src' => array_shift( wp_get_attachment_image_src(
			get_post_thumbnail_id( get_the_ID() ),
			"Full"
		) ),
	'width'   => 60,
	'height'  => 101,
	'classes' => 'thumb'
) );
```
Thanks for your great script. Greetings!