Enhancement: Add case-insensitive cycling

[maxmahlke]

I recently discovered this plugin and I'm really fond of the extension of the c-a and c-x functionality, it's a great idea!

One feature I miss in the definition of the custom cycling groups is to have case-insensitive cycling, e.g.

augend.constant.new({
	elements = { "left", "right" },
	word = true,
	cyclic = true,
        case_sensitive = false,
})

could be used to turn left into right and Left into Right. A more common use case is the true/false tuple, which in some languages like python is True/False.