These methods control how the value of a given node is printed. New methods
can be added if support is needed for a novel class
Usage
tree_label(x, opts)
Arguments
x
A tree like object (list, etc.)
opts
A list of options that directly mirrors the named arguments of
tree. E.g. list(val_printer = crayon::red) is equivalent to
tree(..., val_printer = crayon::red).