Additional support for subclassing of 'treelib.Node'

[RichardHoekstra]

In my own project I am extensively subclassing both 'treelib.Tree' and 'treelib.Node'. Using the 'data' variable, as mentioned in the documentation under advanced usage, does not work well for my use case.

It would make my life easier, and my code prettier, if I can pass additional arguments to Tree.create_node. The dependency injection is already there (self.node_class) and this PR should unlock more of the potential.

treelib/treelib/tree.py

Lines 339 to 346 in 017e891

	def create_node(self, tag=None, identifier=None, parent=None, data=None):
	"""
	Create a child node for given @parent node. If ``identifier`` is absent,
	a UUID will be generated automatically.
	"""
	node = self.node_class(tag=tag, identifier=identifier, data=data)
	self.add_node(node, parent)
	return node