Add aggregate method

[EntilZha]

As part of #38, add an aggregate method.

[EntilZha]

Since the aggregate method is being added to support a LINQ-like interface primarily, I have done some research into what its API should be. Based on the Microsoft documentation for LINQ for one argument, two argument and three argument aggregate, here is what I will be implementing.

aggregate will take either one, two, or three arguments.

• One Argument: This will mimic the behavior in fold_left with the seed value being the first element in the list
• Two Argument: This will mimic the behavior of fold_left
• Three argument: This will mimic the behavior of fold_left with the additional argument being a function to apply to the final result before returning.