Better Dark Mode

- Creates a dark mode of any page by inverting the original colors, background-colors, border-colors.
- Unlike most "dark modes," Better Dark Mode reduces eye-strain by not allowing any RGB values below 50 or above 200 by default. Darkness and brightness limits can be customized by user.

Potential updates:
- Find a way to avoid flash of non-darkened page before extension runs on page load
- Store the active/inactive state as well as darkness/brightness limit variables Chrome-wide (currently using localStorage)