BREAKING CHANGES

• The library was rewritten to use a custom composition (#1)
• Removed rememberTree<T>(), SimpleLeafNode<T> and SimpleBranchNode<T> in favor of the DSL
• The DSL is now the default way to build a tree (it uses the custom composition)
• Added customIcon and customName params on Leaf<T>() and Branch<T>()
• Replaced fileSystemNodes() with FileSystemTree()
• Replaced jsonNodes() with JsonTree()
• Disabled expand/collapse transitions for now (doesn't work yet with custom composition)