Allow merging "scripts" in a deep way

[Fishdrowned]

Just like merge-extra-deep.

Introduce a new merge-scripts-deep configuration setting to control
merging the scripts section of included/required files in a way similar
to to array_merge_recursive() - however duplicate string array keys are
replaced instead of merged, while numeric array keys are merged as
usual.

[Fishdrowned]

To those who want to use this feature but can wait no longer for the merge, please use commit hash in composer.json:

{
    "require": {
        "wikimedia/composer-merge-plugin": "dev-master#96c44b1c"
    }
}