skip_delete for messages

[pandomic]

Currently it is not possible to skip deletion of messages by throwing :skip_delete or by setting corresponding options for #subscribe. It may be very useful, if I, for example, want to decouple messages across multiple subscribers.

[bmidgley]

We have exactly the same need. We want to delete a batch of messages after they are completely processed, not one at a time.

As a workaround, we made an option "auto_delete" on our branch that defaults on.

https://github.com/mavenlink/circuitry/pull/2

It would be nicer if we had this available on message and gave the full message object to the caller. That seems like a refactor.