A wee bit of work on an import command
Also define `WP_IMPORTING`, as this constant can be used to determine whether a number of supplemental actions are run (or not)
First pass at a refactor to make `wp import` more abstracted, and potentially support multiple importers.
Incorporate author mapping support into the WordPress importer.

* If a .csv filename is specified, it will look for an author mapping file.
* If the .csv author mapping file doesn't exist, it will create one for you and wait for you to edit.
* `--authors=create` will create any missing users, and expect that either user_email or user_logins map up directly.
* `--authors=skip` will ignore any byline mapping.
Refactor author mapping code so it can be used with other importers
Make the `--authors` argument required. It's a very necessary step, even if just to have the user confirm they're skipping mappings.