Add configurable file prefix instead of just file:// and txmt://

I'm setup my build box to run metric_fu but none of the file link paths work correctly as they're all generated as file://.

This patch adds a configuration flag link_prefix that I can set to something like http://example.org/files and the links will then become something like http://example.org/files/app/controllers/application_controller.rb instead of file:///users/dj2/foo/app/controllers/application_controller.rb.