Add a progression bar when pulling images

[upils]

A progression bar would be great when pulling images. Currently a user may think darch is stuck.

[pauldotknopf]

Yup. I am using containerd, and their ctr command has a nice progress indicator. It should be just a matter of copy/paste.

https://github.com/containerd/containerd/blob/master/cmd/ctr/commands/content/fetch.go