Add option for displaying long labels for scaled units

For example, display `1.23 Million` instead of `1.23M`.

This should be an option `unit_label` with possible values `:short` and `:long` defaulting to short. It can then be picked up by the different formatters, in the first version just the console formatter to display long names or short names.

edit (@PragTob) : changed to be a general option and not just for the console