Add a member_options option

[ihumanable]

Thanks for the lovely library!

Why

I have a testing scenario where I need to artificially generate partitions in the local cluster to test some functionality.

If I do this then global gets upset and tries to protect me from overlapping partitions. This is a great feature unless of course you are trying to write tests that test partitioning.

What

I can fix this in my tests by starting up the members with additional command line flags, specifically

-kernel prevent_overlapping_partitions false

How

I added an option :member_options that allows a caller to provide additional command line arguments. I'm using it like this

LocalCluster.child_spec(
  {amount, 
   member_options: ~w(-kernel prevent_overlapping_partitions false),
   prefix: prefix})

And this works perfectly for letting me run cluster tests where I need to partition the local cluster.