Add useDarkMode (rehooks#77)

* useDarkMode

Author: donavon

README.md

@@ -103,6 +103,7 @@
 - [`use-socketio`](https://github.com/mfrachet/use-socketio) React hooks to use with https://socket.io/.
 - [`use-substate`](https://github.com/philipp-spiess/use-substate) React hook for subscribing to your single app state (works with your current [Redux](https://redux.js.org/) app).
 - [`use-undo`](https://github.com/xxhomey19/use-undo) React hook to implement Undo and Redo functionality.
+- [`useDarkMode`](https://github.com/donavon/use-dark-mode) A custom React Hook to help you implement a "dark mode" component.
 - [`usePosition`](https://github.com/tranbathanhtung/usePosition) React hook to get position top left of an element.
 - [`useScreenType`](https://github.com/pankod/react-hooks-screen-type) Determining screen size type for Bootstrap 4 grid.
 - [`useScrollSpy`](https://github.com/Purii/react-use-scrollspy) React hook to automatically update navigation based on scroll position.