sprite-position ignoring the group
how we want to interact with sprockets
can find directories in asset pipeline
find image files in the list of #directories

File are overshadowed by similar named earlier in the list of
directories, but all uniq are considered.
can build the sprite
store the images to be accessed from SassExtensions
one runner for every sprite/group
sass: sprite-width, sprite-height
can configure options in config/sprite_factory.yml
raise error when no images given to layout
actually return the sprite paths
provive valid extensions for mini_magick
group name should not break building of name from filename
allow calculation with output from sprite_attr
cache runners between different SASS EvaluationContexts

Else the sprites are generated many times.
sass: sprite-file
can specify output_directory, falls back to public/assets
add digest to output name
generate sprite only when required