Add method to subscribe to multiple storage keys at once

[arjanz]

See #327

Usage example:

def subscription_handler(storage_key, updated_obj, update_nr, subscription_id):
    print(f"Update for {storage_key.params[0]}: {updated_obj.value}")

# Accounts to track
storage_keys = [
    substrate.create_storage_key(
        "System", "Account", ["5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY"]
    ),
    substrate.create_storage_key(
        "System", "Account", ["5FHneW46xGXgs5mUiveU4sbTyGBzmstUspZC92UhjJM694ty"]
    )
]

result = substrate.subscribe_storage(
    storage_keys=storage_keys, subscription_handler=subscription_handler
)