One of the goals is to make the generated content directory directly hostable
via a web browser for performance. This means that the application needs to be
able to generate a file tree identical to the URL structure so that clients can
fetch a single URL and either use the cached image or trigger a manipulation
depending on whether it already exists.
With a url structure of /key and /key/manip for the source and manips respectively,
it becomes impossible to create a file tree matching it since 'key' must be both
the source file and a directory containing the manipulated images.
This updates it to use 'original' as a special manip name so the URL structure of
/key/original and /key/manip can match the directory structure
