Add options for external markup and a prefix for linking assets

[zachesposito]

Hey all,

Thanks for your work on DSS and this task, I've found them very useful. Thought I'd share some options I added to allow blocks to use externally defined markup, since some of my blocks need lengthy example markup and I didn't want them to overtake my style source.

I also added an option for specifying a site prefix that can be prepended to asset references in the template to make deployment to different environments easier.

Zach