Write a function that accepts an integer N and returns a NxN spiral matrix.
> matrix(2)
[
[1, 2],
[4, 3]
]
> matrix(3)
[
[1, 2, 3],
[8, 9, 4],
[7, 6, 5]
]
> matrix(4)
[
[1, 2, 3, 4],
[12, 13, 14, 5],
[11, 16, 15, 6],
[10, 9, 8, 7]
]