Replace fmt.Printf with an actual logger

[ErinCall]

Currently, all logging is done with fmt.Printf (or .Println, .Fprintf, etc.). That's fine for a first pass, but we should really use the log package or something like it. Printf output can pollute the test output, and it would be great to be able to say logger.Debug("...") instead of

if cfg.Debug {
	fmt.Fprint(cfg.Stderr, "...\n")
}

[ErinCall]

Closing this as wontfix—I had succumbed to the temptation to overdesign.

I made some progress using Logrus, but hooking up commands' stdout/stderr to the logger turns out to be surprisingly complex. I don't think the effort is worth the trouble.