add func ParseLevel

[ns5d]

target := log.NewConsoleTarget()
target.MaxLevel = log.ParseLevel(os.Getenv("LOG_LEVEL"))
logger.Targets = append(logger.Targets, target)

[coveralls]

Coverage decreased (-1.0%) to 63.547% when pulling 2d91b381aeaa25ed7204bdb96990d19b27bb342d on ns5d:master into 610cdd147d9aff9523eed107898d2e87ee5336aa on go-ozzo:master.