Support runtime configuration via {:system, _} tuple

[ImmaculatePine]

Currently it is not obvious that one can override MyApp.Cluster.init/1 for runtime configuration unless you read the source code. So, I did 2 changes there:

1. Added support for more idiomatic {:system, _} tuple
2. Added description of both configuration methods to readme

[christianjgreen]

Added support for more idiomatic {:system, _} tuple System tuples are heavily frowned upon

[danielberkompas]

I would prefer to just document that you can use MyApp.Cluster.init for this, rather than support {:system, _} tuples at this point.