Implemented injected-merge-settings

Required by https://github.com/ezsystems/ezpublish-kernel/pull/719.

Adds support for *merged settings* in configuration injection.

Syntax:
```php
array( "injected-merge-settings" => array( 
    "site.ini/ExtensionSettings/ActiveExtensions" => array( 'extension1', 'extension2' ) 
);
```

Order:
- real setting
- injected-setting: replaces the value
- injected-merge-settings: merges the previouys value with whatever is given

*Important: a RuntimeException will be thrown out of a merge value is NOT AN ARRAY, or if one is applied to a non-value (or non existing) setting*