WP-CLI: Store system $USER in author_meta

[westonruter]

While many sites use shared SSH logins, if you're doing it right each user has a separate shell account. When WP-CLI does some action, we should grab the system user who is actually invoking the command, and store it for display.

cf. wp-cli/wp-cli#1158

[ghost]

Wow you're fast.

I was just coming in here to let you know about that php function. Caveat is that it only works for POSIX shells therefore won't work on Windows. In either case, better than nothing.