Create pmprowoo_get_cart_membership_level() function

[ideadude]

abstract from this:
https://github.com/andrewlimaza/pmpro-woocommerce/blob/33b9f6ef97668915665d8561bc485877dff0ac6e/pmpro-woocommerce.php#L382-L395

[andrewlimaza]

This is somewhat related: #148