Clean up `bu`.

The help text is now more thorough. Lines have been split. Directions
are no longer parsed with items and groups.
Change how `do-each` handles commands and directories.

The command is passed in the first arguments. A '--' is passed to signal
the end of the command. After that, directories are passed. There is
also a '-q' option to decrease the verbosity of the script.
Restructure `extract`.

The new structure is a bit cleaner. Also, 'ar' and 'tar'/'xz' archive
support has been added.
Fix units in `growth`.

B -> KB