Allow FLLumberjackIntegrationEnabled and FLDebugLoggingEnabled to be configured externally

The purpose of this PR is that it will allow a project to include FLAnimatedImage to externally define
`FLDebugLoggingEnabled` and `FLLumberjackIntegrationEnabled` via compilation flags, without source changes. This is useful when using FLAnimatedImage as a Cocoapod, for instance.
See https://github.com/Flipboard/FLAnimatedImage/issues/44 for context.

The code works for all projects without any changes.