Quiet flag

[j-maas]

A quiet flag for usage in scripting would be useful. It necessitates discriminating output from logging. For example, the list of tags is output. The report on how many tags were fetched and how many fetched is a log. A practical reason for this is to offer silencing of logging for clean output to feed to other programs.

This could be achieved by configuring the env_logger with more readable default values, except for when the RUST_LOG environment variable is set which should override these settings.