Support for srcset

Hi,

I couldn't use `html-loader` in conjunction but given `ng-cache-loader` has support for resolving URLs, would it be possible to support `srcset` attribute too?

```
<img src="illustrations/hourglass.png"> #works
<img srcset="illustrations/hourglass.png"> # does not work
<img srcset="${require('illustrations/hourglass.png')}"> # does not work but would be great
```
