allow passing multiple items to 'wp plugin update' and 'wp theme update'

also, bring back `wp plugin update --all` and `wp theme update --all`
{plugin|theme} update: improve --dry-run output
{plugin|theme} update: add more examples

[ci skip]
Merge pull request #720 from wp-cli/update-many

Allow passing multiple items to 'wp plugin update' and 'wp theme update'