Add Array#every_nth

Returns the every nth entry in an Array e.g.
[:a, :b, :c, :d].every_nth(2) # => [:b, :d]
[:a, :b, :c, :d].every_nth(3) # => [:c]

I feel like this could be expanded to Enumerables, but am currently
too braindead to find a nice way to do it.