Add darken plugin

[informeai]

Hi ✋🏼 .
Adding the darken plugin which adds colorschema based on terminals in night mode. 👍🏼

[taconi]

Hello, could you update the PR to resolve the conflicts?

Another issue is that your repo.json does not have the Website and License fields, could you add this metadata?

Could you add your repo.json to the plugins/ folder and update channel.json link to use this link?
E.g. https://raw.githubusercontent.com/micro-editor/plugin-channel/master/plugins/editorconfig-micro.json.
Use the plugin name as the file name.

It was missing from the README.md table, could you solve this?