Added the ability to set custom colors for the darkmode (don't have stick to my default!)

Added permissions include tabs and storage and here's a brief explanation:

- Tabs: The way I have the CSS set up, the extension needs to reload BibleGateway tabs if a color changes while biblegateway.com is open. Tabs is used to go through and find any biblegateway tab and reload it.

- Storage: This permission is used to sync color settings across firefox installs. It's how the extension stores colors.

Full source code is available at https://github.com/unquenchedservant/BibleGatewayDarkMode