Convert coalescese_streams function to CoalesceStreamsPreprocessor

[ryan-williams]

coalescese_streams was the last remaining "decorated function Preprocessor", and I couldn't find an example of how to use it.

Here it is converted to be a Preprocessor subclass, like the others. A top-level --coalesce-streams flag is also added, so this now works:

jupyter nbconvert --coalesce-streams --inplace notebook.ipynb

to normalize a notebook's otherwise-nondeterministic stdout/stderr chunking.

For example, a cell might end up with two stderr lines split across two outputs:

{
  "cells": [
    {
      "cell_type": "code",
      "outputs": [
        {
          "name": "stderr",
          "output_type": "stream",
          "text": [
            "before sleep\n"
          ]
        },
        {
          "name": "stderr",
          "output_type": "stream",
          "text": [
            "after sleep\n"
          ]
        }
      ],
      "source": [
        "import sys\n",
        "import time\n",
        "sys.stderr.write(\"before sleep\\n\")\n",
        "time.sleep(1)\n",
        "sys.stderr.write(\"after sleep\\n\");"
      ]
    }
  ]
}

(I usually see this when running notebooks via Papermill)

jupyter nbconvert --coalesce-streams --inplace combines consecutive outputs from the same stream, within each cell:

## inserted before /cells/0/outputs/0:
+  output:
+    output_type: stream
+    name: stderr
+    text:
+      before sleep
+      after sleep

## deleted /cells/0/outputs/0-1:
-  output:
-    output_type: stream
-    name: stderr
-    text:
-      before sleep
-  output:
-    output_type: stream
-    name: stderr
-    text:
-      after sleep

I also implemented this functionality in juq merge-outputs, before noticing this preprocessor!

[blink1073]

LGTM, thank you!