Add light modifier for dark mode

[kripod]

After all the discussion in #69, I decided to create a light palette modifier class for this plugin.

Color replacements were made in the following order:

1. 700 → 300 (main text color)
2. 200 → 700
3. 300 → 600
4. 500 → 400
5. 900 → 200
6. 600 → 300

The light colors were selected with bg-gray-900 and bg-black in mind.

[miharekar]

Since Tailwind 2.0 has first-class support for dark mode it would be great if its plugins would also support it.

So please merge this 😅

[LoganDark]

Bump. Just had to add this absolute monster of a config into my tailwind.config.js file. Was really non-obvious and required me to hunt around a bit. :/

[AlexMeah]

Any chance this could be merged? Currently unable to use this plugin with dark backgrounds.

[adamwathan]

I'm working on a better version of this in this branch (I don't feel bad saying this since this PR is based on my own initial solution 😅):

https://github.com/tailwindlabs/tailwindcss-typography/tree/new-customization-api

So going to close this in favor of that — hope to put something out before the end of next week. Thanks!