--procs takes * nargs using optional_procs choices

Thank you for the feedback here @jph00.

I couldn't quite figure out how to get --procs to work with a tuple.

But using Param(.., nargs="*", choices=optional_procs())="black_format" seems to get something more user-friendly for the command line.

Here are some examples using `raise Exception(L(procs))` in the method to debug it.

```
> nbdev_export -h
usage: nbdev_export [-h] [--path PATH] [--symlinks] [--file_glob FILE_GLOB] [--file_re FILE_RE] [--folder_re FOLDER_RE] [--skip_file_glob SKIP_FILE_GLOB] [--skip_file_re SKIP_FILE_RE] [--skip_folder_re SKIP_FOLDER_RE] [--procs [{black_format,scrub_magics} [{black_format,scrub_magics} ...]]]
...
  --procs [{black_format,scrub_magics} [{black_format,scrub_magics} ...]]  tokens naming the export processors to use. (default: black_format)

> nbdev_export
Exception: ['black_format']

> nbdev_export --procs
> Exception: []

> nbdev_export --procs scrub_magics black_format
Exception: ['scrub_magics', 'black_format']

> nbdev_export --procs black_format
Exception: ['black_format']

> nbdev_export --procs scrub_magics
Exception: ['scrub_magics']

> nbdev_export --procs black_forma
usage: nbdev_export [-h] [--path PATH] [--symlinks] [--file_glob FILE_GLOB] [--file_re FILE_RE] [--folder_re FOLDER_RE] [--skip_file_glob SKIP_FILE_GLOB] [--skip_file_re SKIP_FILE_RE] [--skip_folder_re SKIP_FOLDER_RE] [--procs [{black_format,scrub_magics} [{black_format,scrub_magics} ...]]]
nbdev_export: error: argument --procs: invalid choice: 'black_forma' (choose from 'black_format', 'scrub_magics')

> nbdev_export --procs black_forma scrub_magics
usage: nbdev_export [-h] [--path PATH] [--symlinks] [--file_glob FILE_GLOB] [--file_re FILE_RE] [--folder_re FOLDER_RE] [--skip_file_glob SKIP_FILE_GLOB] [--skip_file_re SKIP_FILE_RE] [--skip_folder_re SKIP_FOLDER_RE] [--procs [{black_format,scrub_magics} [{black_format,scrub_magics} ...]]]
nbdev_export: error: argument --procs: invalid choice: 'black_forma' (choose from 'black_format', 'scrub_magics')

> nbdev_export --procs scrub_magics barst
usage: nbdev_export [-h] [--path PATH] [--symlinks] [--file_glob FILE_GLOB] [--file_re FILE_RE] [--folder_re FOLDER_RE] [--skip_file_glob SKIP_FILE_GLOB] [--skip_file_re SKIP_FILE_RE] [--skip_folder_re SKIP_FOLDER_RE] [--procs [{black_format,scrub_magics} [{black_format,scrub_magics} ...]]]
nbdev_export: error: argument --procs: invalid choice: 'barst' (choose from 'black_format', 'scrub_magics')
```

Author: yegeniy

nbdev/doclinks.py

@@ -127,18 +127,17 @@ def nbglob_cli(
                   skip_file_glob=skip_file_glob, skip_file_re=skip_file_re, skip_folder_re=skip_folder_re)
 
 # %% ../nbs/api/05_doclinks.ipynb 22
-# the import is needed to call getattr on it
-import nbdev.export
-
 @call_parse
 @delegates(nbglob_cli)
 def nbdev_export(
     path:str=None, # Path or filename
-    procs:str="black_format", # whitespace delimited tokens of processors to use.
+    procs:Param("tokens naming the export processors to use.", nargs="*", choices=optional_procs())="black_format",
     **kwargs):
     "Export notebooks in `path` to Python modules"
     if os.environ.get('IN_TEST',0): return
-    if procs: procs = [getattr(nbdev.export, p) for p in procs.split(" ") if p in optional_procs()]
+    if procs:
+      import nbdev.export
+      procs = [getattr(nbdev.export, p) for p in L(procs)]
     files = nbglob(path=path, as_path=True, **kwargs).sorted('name')
     for f in files: nb_export(f, procs=procs)
     add_init(get_config().lib_path)

nbs/api/05_doclinks.ipynb

@@ -347,18 +347,18 @@
    "outputs": [],
    "source": [
     "#|export\n",
-    "# the import is needed to call getattr on it\n",
-    "import nbdev.export\n",
     "\n",
     "@call_parse\n",
     "@delegates(nbglob_cli)\n",
     "def nbdev_export(\n",
     "    path:str=None, # Path or filename\n",
-    "    procs:str=\"black_format\", # whitespace delimited tokens of processors to use.\n",
+    "    procs:Param(\"tokens naming the export processors to use.\", nargs=\"*\", choices=optional_procs())=\"black_format\",\n",
     "    **kwargs):\n",
     "    \"Export notebooks in `path` to Python modules\"\n",
     "    if os.environ.get('IN_TEST',0): return\n",
-    "    if procs: procs = [getattr(nbdev.export, p) for p in procs.split(\" \") if p in optional_procs()]\n",
+    "    if procs:\n",
+    "      import nbdev.export\n",
+    "      procs = [getattr(nbdev.export, p) for p in L(procs)]\n",
     "    files = nbglob(path=path, as_path=True, **kwargs).sorted('name')\n",
     "    for f in files: nb_export(f, procs=procs)\n",
     "    add_init(get_config().lib_path)\n",
@@ -369,10 +369,9 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "`procs` takes the names of any optional processors you with to run on the exported cells of your notebook.\n",
-    "It can be a list of processors\n",
+    "`procs` names the optional processors you wish to run on the exported cells of your notebook.\n",
     "\n",
-    "N.B.: the `black_format` processor is passed in by default. But it is a no-op, unless `black_formatting=True` is set in your `settings.ini` configuration."
+    "N.B.: the `black_format` processor is passed in by default. But it is a no-op, unless `black_formatting=True` is set in your `settings.ini` configuration. You can omit it from `nbdev_export` on the command line by passing in `--procs`."
    ]
   },
   {