Add option for variation group to ignore quantity

[erommel]

Let users specify whether they want variation group to consider quantity multiplier or not.

Use-case scenario example from a user:

Product: Book – $39
Quantity: 1
Optional choices: lifetime updates – $9.99
Final price: 1x Book ($39) + 1x Lifetime updates ($9.99) = 48.99

Problem arises when customer choose quantity of more than 1, because the optional choices (lifetime updates for $9.99) is also calculated two times

Product: Book – $39
Quantity: 2
Optional choices: lifetime updates – $9.99
Final price: 2x Book ($78) + 2x Lifetime updates ($9.99) = 97.98

When choosing quantity 2, it also calculates liteime updates two times instead of just one time.