Enables dry-runs and exclusion by taxonomy term

- pass --dry-run to `migrate author` to preview a migration
- Fixes the function to separate mutliple authors into two distinct terms
- Enables the ability to pass --ignore_term with a comma separated list
  where the first item is the taxonomy to ignore and all following are
  the terms in that taxonomy to ignore. Only works with one taxonomy
  for now.
Updated documentation, a few bug fixes

- Adds documentation for how to use the author migration in the README
- Fixes a bug where authors would not migrate to a custom field
- Cleans out some unused code