Added method for Connect subscriptions

[hassansin]

Added connectSubscription method: https://dev.chartmogul.com/reference#connect-subscriptions

Example:

ChartMogul\Customer::connectSubscriptions("cus_ede41762-02c1-11e9-b8a6-a79ca0ecc71e", [
    "subscriptions" => [
        [
            "data_source_uuid" => "ds_ed13e98e-02c1-11e9-b8a6-4b21284ec572",
            "external_id" => "ext_subscription",
        ],
        [
            "data_source_uuid" => "ds_ed13e98e-02c1-11e9-b8a6-4b21284ec572",
            "external_id" => "ext_subscription_2",
        ],
    ]
]);

[bogdanguranda]

Great!