Change checkboard default colors to full opacity
It would be really useful if the default colors for the checkboard would have full opacity rather than being transparent. Currently, the provided pickers only support a white background, or else the checkerboard pattern for transparent colors is not visible. 

https://github.com/casesandberg/react-color/blob/bc9a0e1dc5d11b06c511a8e02a95bd85c7129f4b/src/components/common/Checkboard.js#L21

For example, in my application I have added some custom styling to the `ChromePicker`, but the dark background leads to the checkerboard pattern becoming invisible. Would be nice to be able to make these kind of style changes, without having to write a completely custom picker 😞 
![image](https://user-images.githubusercontent.com/8808894/114828138-d6a42100-9dc9-11eb-8cce-0ec4f8dc38e4.png)

Love the package btw!
