introduce `pretty` log format

[jeremyandrews]

• introduce pretty log format for --error-format, --debug-format, --request-format, and --task-format

For example, here's a request log "line" that's pretty formatted:

GooseRequestMetric {
    elapsed: 2,
    raw: GooseRawRequest {
        method: Get,
        url: "http://apache/",
        headers: [],
        body: "",
    },
    name: "(Anon) front page",
    final_url: "http://apache/",
    redirected: false,
    response_time: 124,
    status_code: 200,
    success: true,
    update: false,
    user: 0,
    error: "",
    coordinated_omission_elapsed: 0,
    user_cadence: 0,
}