This PR adds a format method to Target, allowing each target to have a different formatter. It also adds reset_format to the Builder to remove the default formatter.

Motivation

When using the with_colors method with both a console-based and a file-based log target, the control codes for the colors are included in the log file. A custom formatter can be used when using TargetKind::Dispatch, but then you lose out on the automatic file rotation that the plugin provides.

Allowing a formatter per target solves this issue with techniques like the following:

Builder::new()
    .reset_format()
    .targets([
        Target::new(TargetKind::Stderr)
        .format(
            move |out, message, record| {
                // custom colorized output here
            },
        ),
        Target::new(TargetKind::LogDir { file_name: None })
        .format(move |out, message, record| {
            // custom non-colorized output here
        }),
    ])
    .build(),