Unify CLI flags and Lassie initialization logic

[kylehuntsman]

Both the daemon and fetch CLI commands have similar flags and use similar Lassie initialization logic. We should dedupe these flags and logic to ensure a consistent experience across both uses of the application.

[hannahhoward]

we have done a decent, if not uniform, job of moving flags that are shared to flags.go in the cmd/lassie directory. However the code to read these flags -- the same ones -- are duplicated across both commands, as is the code to construct lassie from those flags. I'd envision a function in cmd/lassie like BuildLassieFromCLIContext(cctx *cli.Context) *lassie.Lassie that reads the shared flags and makes Lassie from it.

Also, it may make sense to implement this ticket along with #259