Introduce debug groups, for more precise debugging

[danielbachhuber]

--debug is intentionally verbose - if you're trying to debug an issue, you want as much information as you can get.

But, you don't necessarily need all of the information all of the time. We should introduce a second argument to WP_CLI::debug() to permit grouping of debug statements. For instance, we could use WP_CLI::debug( $message, 'bootstrap' ) to group all of the bootstrap debug, and WP_CLI::debug( $message, 'command' ); to group command debug.

Usage would be --debug=bootstrap. Using it as a flag would follow existing behavior of displaying everything.