Use logging levels.

[dpordomingo]

caused by #162::comment.r307651252

I'd transition to using some logging so that we can also filter by level.

Originally posted by @se7entyse7en

[carlosms]

I'm not so sure we should to use log formatted output for a CLI application.

[dpordomingo]

I think it can be useful for debugging and more expert usage, the same way than for other apps.
But we can discuss the output, maybe the standard formats would not fit a CLI tool, so we should use another format.