Within Delta Checker for floats

How about including a Within checker, something like what follows?

Here's an example of it in use: https://github.com/nathany/go-poodr/blob/master/chapter9/gear1/gear1_check_test.go

type withinChecker struct {
  *CheckerInfo
}

var Within Checker = &withinChecker{
  &CheckerInfo{Name: "Within", Params: []string{"obtained", "delta", "expected"}},
}

func (c *withinChecker) Check(params []interface{}, names []string) (result bool, error string) {
  obtained, ok := params[0].(float64)
  if !ok {
    return false, "obtained must be a float64"
  }
  delta, ok := params[1].(float64)
  if !ok {
    return false, "delta must be a float64"
  }
  expected, ok := params[2].(float64)
  if !ok {
    return false, "expected must be a float64"
  }
  return math.Abs(obtained-expected) <= delta, ""
}

# https://gist.github.com/nathany/4238394