[5.x] Add hook into the multisite command

[duncanmcclean]

This pull request adds a hook to the php please multisite command, allowing addon developers to run their own code when a site is being converted from a single site to a multisite.

For example:

// ServiceProvider.php

public function bootAddon()
{
    Multisite::hook('after', function ($payload, $next) {
        // Maybe move some files around...
        // Update a config file or two...

        $this->components->info('Done some multisite magic!');

        return $next($payload);
    });
}

I'm not totally sold on the name of the hook (end). I'm sure we can come up with a better name. Named it after.

[edalzell]

after?