Transparent/RGBA colors

First of all i think this is great 🎆 and thank you @desandro to _make the web great again_ !

## Feature Request (maybe?)
I think that would be cool to have an alpha value that change the color to exadecimal to `rgba()`.

Something like this:

Start by reading the huebee color-picker implementation and how it currently represents hexadecimal colors. Determine where an alpha value would be introduced and define done as supporting transparent/RGBA output matching the requested behavior and image example; no test file is named in the issue.

Written by the indexing model from the issue text.