Add a set of inputs and outputs to a tx.
    Return the new tx on success
    Raise on error

def tx_extend(partial_tx_hex, new_inputs, new_outputs, blockchain='bitcoin', **blockchain_opts):
    """
    Add a set of inputs and outputs to a tx.
    Return the new tx on success
    Raise on error
    """
    if blockchain == 'bitcoin':
        return btc_tx_extend(partial_tx_hex, new_inputs, new_outputs, **blockchain_opts)
    else:
        raise ValueError('Unknown blockchain "{}"'.format(blockchain))