Add an option to sort certain statements to match canonical order

[Hubro]

One of the tedious parts of writing IETF-linted YANG is to keep statements in the correct canonical order.

Add an option (-c/--canonical-order) that will sort certain statements into canonical order.

To begin with, this should only sort statements inside leaf/leaf-list blocks, but it will probably be expanded to sort certain statements inside all blocks, such as must and when.