Add different kinds of custom built-in trees to the library

I think that implementing `DebugTree` as a built-in custom tree inside the library, is a great way to show the users how to properly and efficiently extend and use `Tree` class.

While searching the web for a custom tree, for example, which log his messages into a local file, it seems that there are different implementations which obviously have different implications, because there is no one centralized reliable resource for the specific example custom tree.

IMO, it will be very productive for the users if `trees/` directory will be added at `java/timber/log` directory root path and opportunity will be given to contributes to share their general-purpose reliable custom trees with the rest of users, in `trees/` directory.

Review the Tree API and the proposed DebugTree in the java/timber/log area, then assess how a trees/ directory would fit the library. Define which general-purpose custom trees are in scope and how they should demonstrate extension and use of Tree; the work is done when the agreed examples and their organization are clear and documented.

Written by the indexing model from the issue text.