2.6 insert

Implementation of https://github.com/cakephp/cakephp/issues/1790

But in hindsight I think one could also just manually flatten before passing it into the method:
```php
$result = String::insert(
	'Hello :User.name. You are :Profile.age years old.',
	Hash::flatten($user)
);
```

That would make the additional code unnecessary.
Then I would just close the original issue/ticket.

Or was the original idea something else?