Add baseUrl option for asset resolver

Hi, Thanks for merging the other pull request :)

Here If found another use case of interest: in my app I'm serving the ```/assets``` directory under an specific url (let's say ```http://my.domain/app/assets/```), but the assetResolver transform is replacing all the url(...) links in the css for the url in the asset folder as an absolute url (ie ```/f2e88779520b983cdd117bce5129c80e6f271623/img/masterSprite.png```).

I need to tell the resolver that I'm serving those files in ```http://my.domain/app/assets/f2e88779520b983cdd117bce5129c80e6f271623/img/masterSprite.png``` instead. I think that having an extra option called baseUrl will make the trick, what do you think?

Thanks!
F