Allow loading local HTML/CSS/JS files via the file:// protocol,
enabling Obscura to be used for testing static sites without
running a local HTTP server.
Changes:
- Allow "file" scheme in validate_url() and validate_fetch_url()
- Add fetch_file_url() to read local files and return synthetic
HTTP responses with appropriate content-type headers
- Skip private IP / localhost checks for file:// URLs
Closes#16
