Added a small method to return the last node

[eiso]

First of all thank you for this library, it saved me a lot of time.

I wanted to use treeprint to take a list of file paths and echo it in the same way as the tree command does on Linux.

To make this work, I added a small method to return the last mode of a tree, named, FindLastNode().

	tree := treeprint.New()
	b = r.FilePaths()

	for _, path := range b {

		parts := strings.Split(path, string(os.PathSeparator))

		t := tree.FindByValue(parts[0])

		if t == nil {
			t = tree.AddBranch(parts[0])
		}

		for i := 1; i < len(parts); i++ {
			if i == 1 {
				t = t.AddNode(parts[1])
				continue
			}
			t = t.FindLastNode()
			t = t.AddNode(parts[i])
			t = t.FindLastNode()
		}

	}

	fmt.Println(tree.String())

It is possible the above behaviour could be done before with existing methods in the library but I wasn't able to discover how.

[xlab]

/me slowpoke