Collapse text streams into single output

[frankharkins]

Not sure why, but I've seen the same notebooks executed with different behaviours for streaming text outputs: Some notebook executors collapse streams into a single output, whereas others create a new output object for each stream.

{
 "name": "stdout",
 "output_type": "stream",
 "text": [
  "Hello, world!\n"
 ]
}

Squeaky should normalize this, and it'll have to collapse many adjacent text stream objects into a single object as there's no way of knowing how to split the joined outputs. I also prefer the joined outputs as they're easier to read in the notebook source.

Note: Make sure only to collapse adjacent stdout streams (not stderr etc.).