Improve #8 by adding another `=>` helper to create StateRoute (conditional transition) using array.

This pull request will add `.State0 => [.State1, .State2]` StateRouting-helper (using array) to allow 
only for `0 => 1` or `0 => 2` conditional transition, even when new states are added.

Test Case:
[StateMachineTests.swift#L190-L245](https://github.com/ReactKit/SwiftState/blob/db73c10c1cddd2d2a1d79139a259a328cc8aa40b/SwiftStateTests/StateMachineTests.swift#L190-L245)