extend --led option to support shading LED color by battery level

[taleden]

Expands on the --led option to support more than one color code, creating a palette through which the LED will be faded according to battery level. Also adds support for 3-digit hex color codes (i.e. f0f as shorthand for ff00ff).

If no color codes are provided, a default palette is used, equivalent to "f00 ff0 0f0 0ff 00f". With this palette, the LED would be pure blue at 100% battery charge, cyan at 75%, green at 50%, yellow at 25%, and red at 0%.