Added support for src attributes

Often in IE 8 when you load some background sized images. You need to use AlphaImageLoader. Though it uses `src` attribute to define the resource url like below:

```css
.some-class {
  filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(
    src='images/test.jpg',
    sizingMethod='scale'
  );
}
```

`Grunt-usemin` doesn't support `src`attributes right now. I changed the regex and added some test for it.