Allow `SetLevel` to set a level that mutes all logs (not just up to Fatal)

**Is your feature request related to a problem? Please describe.**
I want to be able to call `SetLevel` and turn off all logging for the logger.

**Describe the solution you'd like**
`SetLevel(level.Off)` or documented method of doing this.

**Describe alternatives you've considered**
I've looked through godocs and issues and cannot find a way to do this with the keywords: off, noop, disable, all

**Additional context**
n/a