Customize reduce message

Nobody has claimed this yet.

The `Part` context manager is in charge of sending the final `reduce` message. But the message
structure is hardcoded; there's no way for the user of `Part` to specify additional data.

We need to give the user more control over the reduce message. Two options so far:

### 1. pass data into `Part` as an argument

```
msg = {'custom': 'hi'}
with Part(..., reduce_message=msg)
pass
```

This has the advantage of being straighforward to understand and a simple implementation.
The downside is that we're forcing the user to construct the message *before* the context block.
It would not allow for constructing the message within the block.

My hunch is that this would be fine for most use cases and probably where we should start.

### 2. send data from the inner block

With some context manager tricks, it is possible for the inner block and the context manager to communicate using `yield` and `gen.send`. The example below proves it *can* be done - but the complexity and obscurity of this approach might be too much.

```python
@contextmanager
def block():
print('before')
msg = yield # wait for a value to be sent
print(f'the inner block sent me {msg}')
yield # run remainder of inner block
print('after')

# Have to instantiate the context manager separate from the with statement
# `with block() as context` will not work
context = block()
with context:
print('inside')
msg = {'custom': 42}
# the inner block MUST send something or we get
# a RuntimeError: generator didn't stop
context.gen.send(msg)
print('still inside the block')
```

which prints

```
before
inside
the inner block sent me 42
still inside the block
after
```

cc @dnomadb @vincentsarago and @sgillies - this context manager / generator hack is a kind of crazy but thought you might enjoy it :-)

Start by locating the Python Part context manager, which currently assembles and sends the final reduce message. Review how the context block and reduce message are connected, then resolve which proposed customization approach the project should support. Done means users can provide additional reduce-message data without breaking existing Part behavior.

Written by the indexing model from the issue text.