v1 plugin changes

Thinking it will be easier to track a single PR rather than 10 separately. 

#### Plugins removed:

- [x] [at2x](https://github.com/reworkcss/rework-plugin-at2x)
- [x] [colors](https://github.com/reworkcss/rework-plugin-colors)
- [x] [ease](https://github.com/reworkcss/rework-plugin-ease)
- [x] extend (already extracted as [rework-inherit](https://github.com/reworkcss/rework-inherit))
- [x] [function](https://github.com/reworkcss/rework-plugin-function)
- [x] [inline](https://github.com/reworkcss/rework-plugin-inline)
- [x] [mixin](https://github.com/reworkcss/rework-plugin-mixin)
- [x] [prefixSelectors](https://github.com/reworkcss/rework-plugin-prefix-selectors)
- [x] [references](https://github.com/reworkcss/rework-plugin-references)
- [x] [url](https://github.com/reworkcss/rework-plugin-url)