Version update

Version update - 2021.1.6

* Added a new `PythonContextMenuItem` class inheriting `ContextMenuItem` which takes python_function and converts it to an executable command.

* Added type hints and argument documentation for most of the methods.

* Added `.create_for`, `.delete` and `.delete_for` methods for `ContextMenuItem` and `ContextMenuGroup`.

* Updated usage example in __main__.

* Updated README.md to reflect the changes.

version upgrade

Now classes ContextMenuItem and ContextMenuGroup are more pythonic.
You can now add items to the ContextMenuGroup during object creation by passing them through items keyword.