Add Dark & Auto color schemes

[JHWelch]

Summary

Fixes #403 Adds support for a colorScheme config value. Possible values are light, dark, and auto.

I left the default value as light trying to reduce the impact on existing users.

~I'm not even a little attached to the colors I used, so please recommend new ones/update them. They are all available as CSS variables.~ Colors updated to match GitHub

Changes to Consider

• I have the "light" color scheme not apply a class to reduce the impact on existing users. I could change to add a class for consistency.
• I ended up using variables for the dark mode colors because it made things much easier, but during that I converted the light mode colors to variables as well. I could revert that change 93103e2c495ed8b926812b021da14a31ef5c2029. However, I think it's a generally useful change because it makes it easier to change the colors in the future, and for users to override the colors in their own CSS.
• Because of the nature of the demo page displaying light, dark & auto, I had trouble figuring out how to build a highlight.js theme that worked. I manually created a css file from the light & dark themes and then adjusted classes as appropriate 0de1e040b06d70e5009490376d8c8bbcd7e44786. I don't think this is an ideal solution, but I couldn't figure out a better way to do it. I'm open to suggestions. Not super familiar with WebPack, but assume we could just build that file dynamically.

General Notes

• Updated README for config & described process for linking two highlight.js themes for dark/light.
• Refactored FileListRender into a class like the other renderers to be able to pass in the config. 4e6bb497880a35ade6454fae100011f53673ce53

[JHWelch]

Left some minor comments. Overall looks ok. Can you also grab the colors from GitHub I think they have a pretty nice color scheme and it would look better.

Sounds good! Will pull the github colors and update.

Updated to github colors in 987d8cb