cli: Add CLI.slop for abstraction of Slop.parse.
cli: Add CLI.slop for abstraction of Slop.parse.

kbsecret's utilities repeat the same Slop.parse incantation over and over,
so this provides a nice abstraction.

woodruffw/kbsecret,kbsecret/kbsecret,kbsecret/kbsecret