added support for getting absolute url for an image when used with <asset:image> tag

I was using asset plugin in my project and needed to render a template in email which used \<asset:image\> tag.

The template created relative urls but since the image was part of the email, it needed an absolute url for google to find it. So I added an additional attribute for getting complete URL of the image.

It can be used like this

 \<asset:image src="myImage.jpg" absoluteUrl="true"/>