Think of youtube-dl but for mangas. And with a very, very small number of supported sites. An alternative with more features is comics-downloader.
Download the current release or use
go get.
$ go get github.com/bake/mdl
$$ mdl -help
Usage: mdl [-format=jpg] [-out=.] [-worker=4] [-progress] url
Flags:
-format string
Encode images as GIF, JPG or PNG (default "jpg")
-out string
Download directory (default ".")
-progress
Show a progressbar
-worker int
Concurrent downloads (default 4)
Version: 1.1.1