Remove upload-attachments WP CLI command and fix upload-directory

`upload-attachments` is pretty useless, as it requires _all_ media files be in the attachment metadata, which isn't a great assumption. REally upload-directory is much more prefered and simpler. ASs it happens, that's be broke for quite some time, so finally fixing it!

Fixes #254, #130, #100, #266, #269, #147