metrics with a target value of zero

`show_best()` and others have code that looks like: 

```r
  if (maximize) {
    summary_res <- summary_res %>% dplyr::arrange(dplyr::desc(mean))
  } else {
    summary_res <- summary_res %>% dplyr::arrange(mean)
  }
```

This should be adjusted for cases when there is a target value. 

Perhaps we can use `maximize = FALSE` and arrange by `(abs(mean - target))`. 

@DavisVaughan  