Full color support: Add support for #RGB hex colors with full range 00-FF for all R/G/B

Colors are a fundamental element of user interface, and user experience. Taskwarrior currently considers it a second class element of communication by forcing users to choose from one of its own colors of liking misnamed as "rgb" 000-555, while the surface it renders on (modern terminal emulators) support full hex range of 0x000000 to 0xFFFFFF.

Each of r,g,b should accept full 00-FF values.

For the terminal emulator "background" color I use, there is simply no decent taskwarrior rgb 000-555 color to choose from (especially but not limited to "color.alternate" in theme). I use #041A3B for "background" in my terminal emulator. It emulates the dark blue hues of night sky, if it makes one curious.

This will welcome people who are not traditional programmers/scientists (who apparently have self ostracized from good color taste) to be part of the system & enrich the user experience by sharing better color palette. Yet again, "color is not a background or foreground", it is an element of communication itself. And form/letter strokes actually flow around it, and assume a secondary role in human psyche.