But `wp post create` can accept `--post_content` as an associative argument. It doesn't really make sense for most cases to pass in a string and then edit it, but it seemed wrong to just discard post content is it were passed as an associative arg. Should we throw a warning of some kind if user tries to use both `--post_content` and `--edit` arguments?

Accepting content from STDIN is an interesting idea, but probably well outside the scope of this PR. It would probably go along with accepting a filename to read content from. What kind of syntax would you use for that?