Add skip entries option

[svartkanin]

This addresses #45

I created a new menu option skip_empty_entries that when set will skip over empty entries in the menu.

Skip-able entries can either be defined with empty string or as None. The new option is set to False as default, so if empty or None values or specified it will fail as it does now. But if the option is set to True it will skip the entries when iterating through the menu. This will allow for grouping menu options together visually.

I've tested it with single selection and multiselection.

[IngoMeyer441]

Thank you very much for contributing! The changes look good to me. I have only some suggestions for minor improvements:

• Declaring the type of a variable without a typing comment is "only" possible for Python >= 3.6. Some people requested Python 3.5 compatibility, so I still use the old typing comments (see the Python 3.5 compatibility check in the GitHub actions).
• I think it is a good idea to both allow None and empty strings for menu separators because empty strings could also be passed from the command line interface. Since None is a special value which is invalid without your new flag, I think we could even always use None as a separator entry. This simplifies the API usage.
• The pre-commit action failed because of a broken black installation... I fixed this in the latest develop, so this check should pass when you push again.
• It would be great if you could add a sentence to the README.md file which describes your new flag. I use the README.md as a documentation replacement. :innocent:

[svartkanin]

Thanks for the quick review and reply. I'm happy to address them, probably towards the end of the weekend :)

[svartkanin]

@IngoMeyer441 I've updated the PR now:

• Added type comments to the declared variables
• Only use None as the separator
• Updated the README.md with an example
• Also in the first version I forgot to add the command line argument for the new constructor parameter, that is included as well now

I had issues running the black commit hook. I kept getting the following error

Traceback (most recent call last):
  File ".cache/pre-commit/repootteniip/py_env-python3/bin/black", line 8, in <module>
    sys.exit(patched_main())
  File ".cache/pre-commit/repootteniip/py_env-python3/lib/python3.10/site-packages/black/__init__.py", line 1423, in patched_main
    patch_click()
  File ".cache/pre-commit/repootteniip/py_env-python3/lib/python3.10/site-packages/black/__init__.py", line 1409, in patch_click
    from click import _unicodefun
ImportError: cannot import name '_unicodefun' from 'click' (.cache/pre-commit/repootteniip/py_env-python3/lib/python3.10/site-packages/click/__init__.py)

I updated the hook to the latest version of black and that resolved the issue

[IngoMeyer441]

Thanks for your changes and your contribution. I tested your code and works fine! I applied some minor improvements to your PR:

• Always consider None as an empty menu entry (independently from skip_empty_entries)
• Consider empty strings as empty menu items if skip_empty_entries is set to True
• Do not require preview functions to accept a None value
• Replace Union[None, x] with Optional[x]
• Sort constructor parameters and cli switches
• Update the documentation of command line switches

I pushed a new commit to the svartkanin-skip-empty-entries branch in this repository. You can apply (and review) the changes with:

git fetch https://github.com/IngoMeyer441/simple-term-menu.git svartkanin-skip-empty-entries
git cherry-pick FETCH_HEAD

If the changes are ok for you, simply push the commit to this PR and I will merge it. Otherwise, please let me know. I am always open for discussion. :slightly_smiling_face:

Thanks again and have a nice weekend! :beers:

[svartkanin]

@IngoMeyer441 thanks for the updates, I've tested things and it seems to work as expected :+1: