Color picker doesn't work

I noticed that VSCode's built-in color picker doesn't work after enabling this extension. The color picker shows the color as a little square next to the CSS value and lets you choose a different color on hover. This feature is pretty useful but unfortunately it's only enabled for the CSS dialects that VSCode natively supports.

Looking at how vetur [implemented this](https://github.com/vuejs/vetur/pull/559), it looks like you have to register with a color provider. This doesn't look exactly trivial, but hopefully it doesn't require too much customization either.