Update slave_post_states immediately after push.

Even though this is more resource-intensive, this helps In case our push process fails halfway through, at least we'll have some
of the site data retained.
Introduce new wp-cli command: push_post

Pushes out posts to the pre-selected sitegroups/sites. More args to be
added later.

see #38
Use a non-protected meta field for source URL

So that we actually syndicate it. Also give it a more descriptive name
(syn_source_url)

See #37
Add missing function declaration to fix fatal.
Store ext_ID for failed post edits

If we error out during an update, retain the ext_ID so that the next
time we try we don't accidentally end up with a duplicate post.