Updating bigclock with RGB color pickers

[joeyhoer]

Updating bigclock to use RGB color values rather than a predefined list of colors for full customizability.

In absence of an official color picker schema solution (https://github.com/tidbyt/community/issues/39), I've added RGB color values as a dropdown to allow for full color customizability rather than forcing users to select from a predefined list of colors. A number schema solution (https://github.com/tidbyt/community/issues/298) would also be superior to a drop down, but those tickets were closed due to inactivity.

I considered using a text field an having the user input a HEX string value for the color, but I decided that input validation would be too difficult. The "number selector" dropdown approach (while inelegant from a configuration UI perspective) ensures that the color will always be valid.