RGBA hex value support

[seanmiddleditch]

The library is great!

The only real issue I've run into at all is that I need RGBA hex, e.g. getting 8-digit #rrggbbaa format output, which the library does not support natively. This form can be generated manually using the component rgba output, of course, but the UI itself has some limitations as well.

Notably, ChromePicker will only show the hex input option if alpha is 1.0. This looks like it might be an issue for several other picker styles that display hex values.

An option to get hex output with alpha included, and to enable hex input for field controls, would be phenomenal. I understand this may not perfectly match the picker styles this library attempts to emulate, but may be an area where it can improve over the original inspirations.

Maintaining compatibility could be achieved by either adding a flag (hexWithAlpha: boolean = false?) or adding a new set of colors/views (hexa?).