Fixes #645 - adds bottom to top `media` merging.

Fixes #645 - adds bottom to top `media` merging.

In addition to top to bottom media merging we now support the reverse
order one. It works similar way as same selector two-way merging - see
2f8f100592908225c853997051f56bb87c35c20d.

Consider this example where top to bottom is not possible but reverse is:

```css
@media (max-width:768px){.one{padding-right:0}}
.one{padding:10px}
@media (max-width:768px){.one{margin:0}}
```