Cleans up help text, adds author sub command
It works!

- `wp migrate author` will migrate all post authors to a taxonomy
  called 'author', assuming such a taxonomy exists. If one doesn't
  exist, you can pass the <type> parameter
wp randomize, that's a thing now

- wp randomize taxonomy <taxonomy> will attach random taxonomy terms
  to your posts. Great for when you need categories but aren't sure
  what they should be yet. IF you are sure what they should bue but
  not necessarily which posts should get them, pass the --terms=
  flag and list out some terms separated by commas.