Ability to set level only if one is not persisted

[Mr0grog]

In environments where a logging level has been explicitly set and persisted (usually a developer’s browser), it’d be nice to be able to leave the logging level alone, while otherwise giving a default logging level to use if one hasn’t been persisted. The only way to accomplish this right now is to explicitly peek into local storage or cookies to see if a value is set, which isn’t very nice.

I’m thinking this could either be a third argument to setLevel or, probably clearer, a separate method, e.g. setDefaultLevel.

[Mr0grog]

I'm happy to do a PR for this if you'd like, too.