Feature request: single callback with all the values #92

I see a need for an ability to get a callback with all the values in a multi-read or multi-write request at once. The callback would then return a list (or iterator) of values in the case of a read operation.

In my case I have a situation where typically 2 adjacent modbus registers map to the high/low parts of an internal parameter, and I want to be able to handle writes atomically and send data from the same point in time for reads.

I don't see an obvious way to add this functionality, though, and right now I inject an alternative implementation of WriteMultipleRegisters.execute() that doesn't have the loop but calls the callback immediately instead.

This is related to #73, and would provide a more generally useful interface (IMHO).