Merge themebg into master to fix background option

[RedFantom]

As shown in the image, the commits in this branch fix the usage of the background kwarg (renaming it to themebg to avoid confusion with the default background kwarg of Tk). In addition, the commits implement modified versions of the config, configure and cget functions to set and retrieve options. The custom options of ThemedTk can be set upon initialization, unlike the other options of Tk, but now also during run-time.

[Akuli]

Does your config and cget stuff work with tkinter's widget['option'] syntax? I think I have tried to add my own options to widgets, and I also needed to override __setitem__ and __getitem__ to make it work. It might be easier to add the option with Tk instead of tkinter, if that's possible.

[RedFantom]

You're right! Thank you for pointing that out, @Akuli .