Use new after_wp_config_load early invoke to run wp db * commands - #57

Landing this will first require merging wp-cli/wp-cli#4488

@gitlost I think #57 needs to land first (I've restarted that build), then I'll tag a release for wp-cli/db-command, and then we can update composer.lock to pull in the latest.

@gitlost Sorry, meant to leave this message on wp-cli/wp-cli#4490

@wp-cli/committers Ok, build is passing here and this is ready to land.