Add support for pixel density in asset breakpoint object

[weotch]

From previous asset docs:

---

You an also specify pixel density constraints:

:image='{ 500: 'image-mobile.png', '500@2': 'image-tablet-2x.png' }'

[weotch]

An easy way to implement this is to map through the config before it is processed, parsing the @ delimiter out and multiplying the left side by the right side. And then, when reading the current width in deciding which image to show, multiple the screen width by the pixel density.

[weotch]

We'll get this via srcset in 2.0.