added ability to replace the base value with a different string.

I´ve added a `base_replace_with` option. In my project layout I have my templates in src/app/html directory and my development build process places them into the /template folder. I wanted to leave my template names in my angular routing code as "templates/name-of-template.tpl.html" so that I could continue to use standard lazy loading templates with the development build, but the template cache with the production build. Unfortunately I was unable to achieve this using the standard `base` option.