Add normalized RGB values, e.g, 0.1,0.2,0.3, parsing

[phqb]

I need a CLI tool to display RGB colors from normalized format which is used in GLSL shaders code. Then I came across this fantastic tool.

The parse_numeric_rgb() in this repo parses RGB values as double and always divides them by 225 before constructing Color.

This PR add a parse_numeric_rgb_normalized() in which if RGB values are all in range [0,1], then they are considered normalized and used as-is to construct Color.

parse_numeric_rgb_normalized() has higher precedence than parse_numeric_rgb() to workaround the ambiguous values like 0.01 0.01 0.01 could be a rounding error of black color. I think making parse_numeric_rgb() parse values as u8 solves this ambiguity problem, but it is not backward compatible and breaks test cases.