Added optional prefix to cache key

Hi,

I use a workflow where during templates are loaded individually as html, and they're only packaged using html2js in release mode. For reasons specific to my setting, the template urls are also relative. To make it possible to add cache keys without the leading "/" (i.e. similar to relative url) during html2js I had to add an optional custom prefix that gets added to the cache key.

Not sure if this makes sense for anyone else but here it is.