extracted create_workers_by_format from ImageOptim#initialize
extracted create_workers_by_format from ImageOptim#initialize

weheartit/image_optim,weheartit/image_optim,toy/image_optim,mihailim/image_optim,toy/image_optim,mihailim/image_optim,toy/image_optim,ramiroaraujo/image_optim,ramiroaraujo/image_optim,mihailim/image_optim,ramiroaraujo/image_optim,weheartit/image_optim