Add support for binary and metric byte prefixes

Most people (marachino included) incorrectly combine metric byte prefixes with base 1024 units. The hard drive industry correctly uses metric prefixes with base 1000 units, causing confusion. This pull request standardizes maraschino's units and adds an option to the disk space module to choose between them.

Previously a 24 TB group would have been incorrectly reported by maraschino as being 21.8 TB, now it can either be reported as 24 TB or 21.8 TiB depending on the config setting.

Note that the changes to format_number mean that anywhere it is used will now use the correct units.