Feature Request: Control how verbose output is

Nobody has claimed this yet.

As described in https://groups.google.com/forum/#!searchin/got-your-back/verbose%7Csort:date/got-your-back/50TDppD3-5E/971Z6SwtFwAJ, it would be helpful to have optionally quieter output.

Full steps to reproduce the issue:
1. Run GYB (especially as a cron job)
2. See many lines showing all updates

Expected outcome (what are you trying to do?):
I'd like to optionally see less output. I'd think that a numeric `--verbose=<#>` argument would cover what I'm thinking of. The levels that occur to me are:

0. Silent except for fatal errors
1. Warnings but no other information
2. Everything from lower levels and statistics on completion of a step but not status messages during the step
3. Everything from lower levels and status messages during the step (this is the current output)
4. Everything from lower levels and the current output from `--debug`

Start by tracing GYB's command-line argument handling and the code that emits the current status, statistics, warnings, fatal errors, and --debug output. Review the linked discussion for context on quieter output. Done means a numeric --verbose level supports the five described output tiers, including cron-friendly silent and warning-only modes.

Written by the indexing model from the issue text.