Add option to invert logic of the button icons

I'd like to have the sun displayed if the light theme is active, so the sun will show to indicate that it's the light theme and if the dark theme is active I'd like to show the moon.

The current implementation is showing what the button will do but the described behaviour feels more natural IMO. Both could be added so it can be changed with a prop.

To make it more clear, I've created a [sandbox](https://codesandbox.io/s/react-theme-toggle-button-olr5g) with the modified button code.

If you like I could create a PR for this.