Add valid_send_options/1

1.5.1

Add valid_send_options/1 (#12)

Add an optional :offload send mode

This optional mode offloads the send/2 calls to the remote Manifold nodes
to one of the local Manifold.Sender processes.

This is experimental. Details is in the README. Use with caution.

speed increases + support for sharded partitioner

1.3.0 includes performance improvements in #3, and the ability to shard partitioners in #4.