Allow more verbose logs (option 1)

[tonglil]

Closes #20.

It looks like @thockin made a PR to Zerolog to allow arbitrary levels. I updated this library to allow these more verbose levels in 2 approaches.

This is similar to https://github.com/go-logr/zapr#increasing-verbosity.

---

I prefer this approach, where the specified Zerolog level matches logr's V() levels numerically.

This is clear that setting Zerolog to named levels shows level names, while numerical levels (-3) shows their "negatively corresponding" V() logs (V(3)):

	l := zerolog.Level(-3)
	zerolog.SetGlobalLevel(l)
	zl := zerolog.New(os.Stdout).Level(l)
	log := zerologr.New(&zl)
	log.Info("Info you should see this")
	log.V(1).Info("v1 you should see this")
	log.V(2).Info("v2 you should see this")
	log.V(3).Info("v3 you should see this")
	log.V(4).Info("v4 you should NOT see this")

Yields:

{"level":"info","v":0,"message":"Info you should see this"}
{"level":"debug","v":1,"message":"v1 you should see this"}
{"level":"trace","v":2,"message":"v2 you should see this"}
{"level":"-3","v":3,"message":"v3 you should see this"}

[hn8]

@tonglil Option 2 is less confusing. Otherwise, zerolog level -2 is not mapped to any logr level. Since zerolog.TraceLevel is -1, -2 would be next level someone might try first.