http_generated_images_path

Hi,

You are missing `http_generated_images_path` option. Existent `generated_images_path` does not honor relative path to generated assets.

Example:
`http://localhost:8080/src/img/icons-s257c79e80d.png` (wrong path with `generated_images_path` set to /src/img. Otherwise it places generated images to wrong destination.)
vs
`http://localhost:8080/img/icons-s257c79e80d.png` (correct, with http_generated_images_path set inside config.rb)

Start by tracing how generated_images_path is read and applied in config.rb, then compare the documented wrong and correct asset URLs in the issue. Done means supporting an http_generated_images_path option so generated asset URLs omit the unwanted relative path; no test file is named.

Written by the indexing model from the issue text.