Solve Problem 2

* Use elixir pipe `|>` operator to solve problem
* Returns fib sequence in a list with `fib_numbers/1` and
  `fib_numbers/2`
* Appends next fib sequence number with `append_fib_number/1`
* Returns sum of even numbers in a list with `sum_of_even_numbers/1`

https://projecteuler.net/problem=2