[isort] First implementation based on code_prettify

Actually I didn't add keyboard shortcuts because I can't find any
combination that works on my machine.

Author: Benjamin Abel

src/jupyter_contrib_nbextensions/nbextensions/code_prettify/README_isort.md

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+# Sort imports using isort
+
+This nbextension sorts imports in notebook code cells.
+
+Under the hood, it uses a call to the current notebook kernel to reformat the code. The conversion run by the kernel uses Python's package [isort](https://github.com/timothycrosley/isort) by [Timothy Edmund Crosley](https://github.com/timothycrosley).
+
+The nbextension provides
+
+- a toolbar button (configurable to be added or not)
+
+**pre-requisites:** of course, you must have the corresponding package installed:
+
+```
+pip install isort [--user]
+```
+
+## Options
+
+All options are provided by the [KerneExecOnCells library](kernel_exec_on_cell.js). There are a few nbextension-wide options, configurable using the [jupyter_nbextensions_configurator](https://github.com/Jupyter-contrib/jupyter_nbextensions_configurator) or by editing the `notebook` section config file directly. The options are as follows:
+
+- `isort.add_toolbar_button`: Whether to add a toolbar button to transform the selected cell(s). Defaults to `true`.
+
+- `isort.button_icon`: A font-awesome class defining the icon used for the toolbar button and actions. See [fontawesome.io/icons] for available icon classes. Defaults to `fa-sort`.
+
+- `isort.show_alerts_for_errors`: Whether to show alerts for errors in the kernel calls. Defaults to `false`.
+
+- `isort.button_label`: Toolbar button label text. Also used in the actions' help text. Defaults to `Sort imports with isort`.
+
+- `isort.kernel_config_map_json`: The value of this key is a string which can be parsed into a json object giving the config for each kernel language.
+
+## Internals
+
+Under the hood, this nbextension uses the [kerneexeconcells library](kernel_exec_on_cell.js), a shared library for creating Jupyter nbextensions which transform code cell text using calls to the active kernel.
+
+See the [shared README](REAME.md) and [kerneexeconcells library](kernel_exec_on_cell.js) for the internal model used by the nbextension.

src/jupyter_contrib_nbextensions/nbextensions/code_prettify/isort.js

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+// Copyright (c) Jupyter-Contrib Team.
+// Distributed under the terms of the Modified BSD License.
+// Authors: @benjaminabel
+// Based on: code_prettify extension
+
+define(['./kernel_exec_on_cell'], function(kernel_exec_on_cell) {
+    'use strict';
+
+    var mod_name = 'isort';
+
+    // gives default settings
+    var cfg = {
+        add_toolbar_button: true,
+        register_hotkey: false,
+        show_alerts_for_errors: false,
+        button_icon: 'fa-sort',
+        button_label: 'Sort imports with isort',
+        kbd_shortcut_text: 'Sort imports in' // ' current cell(s)'
+    };
+
+    cfg.kernel_config_map = { // map of parameters for supported kernels
+        "python": {
+            "library": [
+                "import isort",
+                "import json",
+                "def _isort_refactor_cell(src):",
+                "    try:",
+                "        tree = isort.SortImports(file_contents=src).output",
+                "    except Exception:",
+                "        return src ",
+                "    else:",
+                "        return str(tree)[:-1]",
+            ].join('\n'),
+            "prefix": "print(json.dumps(_isort_refactor_cell(u",
+            "postfix": ")))"
+        }
+    };
+
+    var converter = new kernel_exec_on_cell.define_plugin(mod_name, cfg);
+    converter.load_ipython_extension = converter.initialize_plugin;
+    return converter;
+
+    });

src/jupyter_contrib_nbextensions/nbextensions/code_prettify/isort.yaml

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+Type: Jupyter Notebook Extension
+Name: isort formatter
+Description: Sort imports in python files using isort
+Link: README_isort.md
+Main: isort.js
+Compatibility: Jupyter 4.x, 5.x
+Parameters:
+
+- name: isort.add_toolbar_button
+  description: Add a toolbar button to convert the selected cell(s)
+  input_type: checkbox
+  default: true
+
+- name: isort.button_icon
+  description: |
+    Toolbar button icon: afont-awesome class defining the icon used for the
+    toolbar button. See http://fontawesome.io/icons/ for available icons.
+  input_type: text
+  default: 'fa-sort'
+
+- name: isort.button_label
+  description: Toolbar button label text
+  input_type: text
+  default: 'Sort imports with isort'
+
+- name: isort.kernel_config_map_json
+  description: |
+    kernel_config_map_json:
+    json defining library calls required to load the kernel-specific
+    converting modules, and the prefix & postfix for the json-format string
+    required to make the converting call.
+  input_type: textarea
+  default: |
+    {
+      "python": {
+        "library": "import json, isort\ndef _isort_refactor_cell(src):\n    try:\n        tree = isort.SortImports(file_contents=src).output\n    except Exception:\n        return src \n    else:\n        return str(tree)[:-1]",
+        "prefix": "print(json.dumps(_isort_refactor_cell(u",
+        "postfix": ")))"
+      }
+    }