Add search-replace extension for multisites

[JoosuaKoskinen]

What are the main changes in this PR?

Extends wp-cli wp search-replace to replace DOMAIN_CURRENT_SITE on multisites when needed.

Prevents multisites from breaking on automatic domain changes or manual search-replace.

$ wp option get home
https://joosua.fi
$ grep -F htdocs/wp-config.php -e DOMAIN_CURRENT_SITE 
define( 'DOMAIN_CURRENT_SITE', 'joosua.fi' );

$ wp search-replace 'joosua.fi' 'wordpress.joosua.me' --quiet
$ wp cache flush
Success: The cache was flushed.

$ wp option get home
https://wordpress.joosua.me
$ grep -F htdocs/wp-config.php -e DOMAIN_CURRENT_SITE 
define( 'DOMAIN_CURRENT_SITE', 'wordpress.joosua.me' );

[sjaks]

Not affected:

Affected:

Seems to change the domain to correct value so that the correct domain is used everywhere.
Dry-run and regex exclusions work and a git commit is created for changes.

LGTM!

[elguitar]

LGTM!

How should we document this? Currently the user will not expect that the search-replace will do stuff that it would not normally do.