Allow DebugTree to be subclassed

[NachoSoto]

I might be missing something, but I can't find a way to implement a CrashReportingTree like in the example that implements TaggedTree instead of Tree.

As a consequence of this, logs sent to Crashlytics don't have the context of the tag associated to them, which make them pretty useless.

It would be great if the tagging functionality implemented in DebugTree was abstracted out so that other subclasses could reuse it, or if the implementation of the Tree methods weren't marked as final so that they could be overridden to forward logs to Crashlytics instead (or anything else).