Accept "filesystem" as a value for SITE_URL
When building a generic template, we may need to use absolute URLs. If we can use `SITE_URL` in production, it can cause problems when building in development. By setting `SITE_URL` to `filesystem`, the URL will be replaced by the filesystem output directory (e.g. `file://path/to/output`) so the browser will open the correct file.
Showing with 5 additions and 2 deletions