Add SetLevel() to DefaultLeveledLogger

[mjmac]

Also updates DefaultLeveledLogger implementation to conform. Allows
a custom LoggerFactory to update a logger's loglevel at runtime.

[hugoArregui]

Do we need it in the interface? Not sure how you end up implementing the factory but it seems to me that you could add it to the DefaultLeveledLogger but not to the interface.

[mjmac]

Do we need it in the interface? Not sure how you end up implementing the factory but it seems to me that you could add it to the DefaultLeveledLogger but not to the interface.

I have to conform to the LoggerFactory interface in order to pass in my custom factory, and that interface definition uses the LeveledLogger interface:

type LoggerFactory interface {
NewLogger(scope string) LeveledLogger
}