Extend existing color picker to support alpha channel

[alexist-ong]

Hi Devin,

I recently had this idea of extending the colorpicker to support more color code formats, especially the alpha channel.

I did some study and implemented wp-color-picker-alpha.js from https://github.com/kallookoo/wp-color-picker-alpha to the existing colorpicker. I hope the changes I made are okay to be merged into your repository.

Please let me know if you have any comments on the above. Hope to hear from you soon.

Regards.