Feature Request: Support `init` argument for reduce functions.

[schlichtanders]

While DTables.jl supports the init for mapreduce officially in its documentation, DArray has no documentation about its reduce functionality, and those existing in the code do not support the init keyword.

It would be great if also DArray.jl supports init for the reduce functions to efficiently run something like OnlineStats over them.

I've been looking at this file for the current implementations.

[jpsamaroo]

Thanks for the report! The entire DArray map/reduce functionality is a bit of a mess - I'll clean it up and then plumb through init. I can add some tests for OnlineStats specifically - it's certainly something I expect users to be able to use reliably.