Better logging

[jonjohnsonjr]

We currently use the standard library's log package, but that has some issues:

https://github.com/google/go-containerregistry/issues/266
#352

There are some messages we'd like to get to users because traditionally they've been very useful in diagnosing problems. The way we're using log means that there's no way to turn that off.

One approach would be to log nothing, but expose issues through some diagnose command:
#246

We could also use a no-op logger by default that just discards messages, and register a real logger in our own binaries: https://golang.org/pkg/log/#New

We could also have a verbose logger that is useful for debugging that gets turned on via some flag.

[ekcasey]

+1

For consumers of this library it would be nice to have control over the output without having to mess with the standard logger.